Here is a modification tutorial for the shorts that I posted here for my 27 dresses series.
1. For this tutorial you need to start with a pants or shorts pattern, preferably one that you've used before, and it should probably be a low-rise fit...I'm sure you could use a higher rise but I didn't so I'm not like absolutely sure it would work the same way.
2. For fabric, I would say you need around a yard, I normally would use less for a pair of shorts but I would guess that it depends on the size you make, the width of the fabric, etc, so just use discretion. You could even follow the instructions to make the pattern and then lay them out to see how much you will need before buying/choosing your fabric.
3. These shorts are lined. I'm sure you could make them unlined, but I didn't want topstitching by the ruffles so I knew they had to have facing, so I just went ahead and lined them. Any lightweight fabric will do.
4. 2" elastic was used for the waistband. You need whatever length is comfortable on your waist. You could do a drawstring if you like, or narrower elastic, but keep in mind that if you make the casing smaller you need to make sure that the rest of the shorts has enough fabric to give you the rise you want - more on this in the instructions.
5. Paper for the pattern
Directions for pattern:
On the top corner, extend the waist out a couple of inches. Because the front and back overlap, this isn't a critical measurement.
Draw a nice curve from the point of the line you just drew around to meet with the hem (it can cut off the lower corner, that's fine). How big or small of a curve you make is up to you. It's not like I pulled out a compass or anything.
You want the top to be a right angle, so place something with a right angle lined up with the top and line up the other side of the angle with a ruler that extends to the curve below.
Now about the waist casing - no matter how big of a casing you have, what you do is to the top, add the width of the waistband that comes with the pattern, not including the seam allowance (if your pattern doesn't have one you don't add any), and add the width of the desired casing over that. The pant piece also comes with SA but just leave that on, because you need extra at the top when you fold over the casing to sew.
For me, my pattern came with a waistband that was two inches wide when you take off the SA. I want a two inch waistband, so that makes four inches that I need to add to the top.
Without moving the ruler, remove the right angle, and draw a line along it from the curve at the bottom up to the top, and then continue it up the top for the width determined just before (in my case, four inches)
You need to draw a line from the place where the back seam curves up to the line that you made at the top. they need to meet at a right angle.
What it should look like.
One thing you can do to ensure a good fit is to measure on a comfy pair of pants the length from the top of the waistband to the seam at the crotch along the center back, and compare it to the length here, not including SA or the width of the desired casing (2" for me).
If you need more, you can add it most easily at the bottom like shown. I didn't add any, this was just drawn on the computer. I probably could have used it, but my shorts turned out comfy anyway. This is just to be on the safe side.
Lay the finished back piece over the front, so that the original top corners lay over each other. Trace over the outside and top, then slide it over so that it lines up with the center front, and trace that.
I decided to cut some off of the bottom because the inseam needs to remain the same length.
To try to make the back and front more even, I shaved some off the curve on the back piece. As you can see, I'm not using any kind of super exact equations to come up with these. You just want to make sure the pieces look relatively normal, the front and back sides are somewhat even, and the inseams are the same length, which they are in the pattern so you just have to be careful not to change them.
Cut two of each in fabric and lining, except make the fabric twice the width of your casing shorter (so I made it four inches shorter). I used the dress for the fabric, and I just barely had enough width for the pieces. You can use whatever, I guess, just make sure you have enough before you cut into it obviously.
You also need to measure around the outside of the front and back five inches from the top edges (or one inch more than twice the width of your casing). Multiply that by 1.5. You have to cut 1 1/2" strips to make this length for the ruffle twice, one for each length.
1: If you have multiple small strips, sew together to make the two long strips. Press seams open. At the ends, fold right sides together and sew the end closed. Trim the top and turn right side out. Press along the entire length and baste the raw edges together.
2: Right sides together, sew front to back at inseam for both legs (if your fabric looks the same on the front of it and the back, take care not to make them the same). Do the same for the lining.
3: Ruffle the strips so that they fit onto the sides and bottoms of the legs. I did this by hand. If you end up making them too short, that's fine, since the front and back overlap and you can just take it from the back and it won't show.
4: Ease-stitch around the curves of both fabric and lining fronts and backs, and gather just a tiny bit, not so much that it will actually look gathered, but it should form a little bit of a bubble I guess, while the fabric by the stitching is still smooth. This will prevent the finished shorts from looking stretched out around the curve when it's finished. I didn't actually do this when I sewed them, but I had to do some fancy steaming when I was done to make it look right and would have saved myself a lot of work.
5: Pin the ruffle to the shorts five inches from the top edge (or twice the width of your casing plus one inch) and around to the back on the right side. Baste. Pin the lining right side down to that, lining up the inseam (the top of the lining won't line up with the top of the fabric, it will be shorter), and sew along the baste line. Trim and turn.
6: Right sides together, pin the crotch seam together as shown, and sew, being careful not to catch the lining. If you're doing a regular casing with elastic, leave an opening at the top back big enough to fit your elastic through plus 5/8".
7: Pull the lining upwards and do the same as in the last step.
8: Turn right side out by sticking your hand through the tube formed at the crotch and pulling it through.
9: Lay one back piece face up on your work surface. Lay the corresponding front piece face down over that, and slide over so that there is about an inch and a half from the raw edge on the front (on top) to the raw edge of the back (folded under in the picture). The exact measurement isn't critical as long as it's the same on both legs. Make sure it's loose enough to be comfy, though, of course.
10: Pull the lining out of the way and pin the layers of fabric together.
11: You're going to start sewing directly over the stitches that hold the facing, ruffle and fabric together. Backstitch at least a half an inch down from the top edge of the facing, then sew straight up to the top of the fabric, being careful not to catch the facing.
It will look like this on the front:
Do the same for the other leg.
12: Sew the waistband - I did not do a regular casing, I folded the fabric around the elastic and sewed through both. I personally think this makes a more professional-looking waistband. This is the easiest way I have been able to do it:
Sew the elastic together at the ends, then fold in half with that stitched part at one end, and mark the other end with a pin. The place where this pin is will be pinned to center front later. Now fold so that that pin is on top of the stitched part, and pin each of the folded ends - these are where it will be pinned to the side seams. So you have a circle of elastic marked into quarters by three pins and stitching.
Starting at center front, fold 5/8" fabric down over elastic at place where the stitching is on the elastic, then fold over so that the fabric wraps relatively tightly around elastic. Pin at lower edge. To the at the center front with the corresponding pin in the elastic; remove the pin when you pin the fabric down, though. It isn't fun to trap pins inside sewing - I have done it! Do the same for the two side seams. Be careful not to twist the elastic.
You should try it on inside out at this point to make sure you like the rise. If you want to lower the top, you have to unpin the sections you want to lower, leaving the pins marking the sections for last (mark those again on the elastic). Then you can re-pin, increasing the 5/8" underneath be the amount you want off the top when you do the place marking the sections. When you stretch out the sections, it will basically even itself out, just try to make sure everything lies smoothly. I have a slight swayback so I ended up taking some off the front to compensate, even though it's a lot of work to re-pin it and all.
Sew 1/8" to 1/4" in from the lower edge of the elastic, stretching until the fabric is smooth as you go. If you sew on the outside, you can use the pins as a guide, or sew the width of your elastic minus 1/8" to 1/4" from the top. I personally just feel the elastic as I go on the outside to make sure I catch it, and don't measure anything.
For a casing
If you are making a drawstring, you need to first make buttonholes at the center front, in the middle of wherever your front casing would be. The center of the buttonhole should be one and a half times the width of your casing plus 5/8" down from the top, with two one each side of the center front seam.
Fold and press 3/8" (leaves 1/4" for room for stitching) towards the inside all around the top. Fold and press again the width of your casing plus 1/8" to 1/4". There is technically only 1/8" there extra, but taking another 1/8" out of the main part of the leg is not going to effect the rise very much. Pin; if you can pin it in such a way that the pins are on the outside and you could stick your elastic in and try it on, you should, cause now's the time to see if maybe you want the rise a little lower in front or anywhere. Sew the fabric down close to the edge. Make it as close as possible. There is the 1/8" to 1/4" that you folded down extra as room for the stitching.
If this is a casing for elastic, There is that opening at center back to thread the elastic through. Thread it through, leaving the ends out, and sew the ends together. Pull the sewed-together ends into the casing and stitch over the opening to close it.
If there is a drawstring, just pull it through the buttonholes and tie the knots at the ends. I stitched a little mock-drawstring to the center front that was a tie thing on the dress that I got the fabric from.