I started by tracing my best fitting RTW (ready to wear) pair. I used an old t-shirt to sew a muslin from this pattern and they were two sizes too small. I kept adjusting the pattern until I got what is pictured here. I don't add seam allowance to the edges that will get elastic, but I add 1/2-inch SA to the connecting seams.
For reference, I wear a Hanes size 7 underwear. I can make one pair of unders from a Hanes size small (boxy/classic fit) t-shirt.
- I try to avoid VPL (visible panty lines), so I wanted the only seams to be on the sides, not across the bottom where my pants pull tight. All the tutorials I read have the front and back connected where they are in RTW pairs, but I eliminated this seam.
- Underwear always crawls up on me. Always. I decided to experiment and found out that if the cheeks are fully covered, that is, the elastic goes around, not over, them, then the unders stay in place. It's the same idea as having bra underwires cup the breasts instead of riding up over them (well, there is no support in the elastic, but the cupping visual is the same). Try finding full coverage, low-waisted unders in the stores, especially if you don't wear white. I don't think they exist.
- The crotch in this pattern is a little wide, but it is easy to free-hand it ½-inch or so narrower if I feel like it. If your unders elastic normally pinches you, try making the crotch area wider or the elastic looser in this area.
- I zig-zag the crotch layer wherever it does me the most good, which isn't where it is placed in RTW pairs. You can also experiment with how long you want this layer. I haven't found this small line of stitching to make a visible line under clothing. I put the insert in after the side seams are sewn, but before the elastic is on. I find it helps with accurate placement to have the unders assembled before sewing on the layer.
- I want my unders waistband to be lower than any other waistbands I might be wearing. This is especially important if I'm wearing shorts under a skirt. For comfort, all the waistbands need to be staggered. With some experimenting, I got the back waistband to be high enough to be comfortable, but low enough to not be above my jeans waistband. The front waistband is much lower and nicely curves under my belly and lower than any of my clothes waistbands. To get this, play around with a curved ruler, free-handing curves, and know that you might have to trim off small points at the side seam after the elastic is attached.
I've made this pattern with t-shirts and yardage knit fabric. I'm anxious to get my hands on some stretch satin to see if the pattern needs to be adjusted for woven fabrics.
Now that I've gotten this pattern the way I want it, I bet it would work well for swimming suit bottoms, too.
Hope these ideas help. Once you get the pattern right, sewing unders is one of the quickest, easiest projects!