I've been intending to do this for over a year, and finally got the courage to do it over the last week. I replaced the waistband in my Sweet Legs Plus leggings I bought last fall (I think they were called Dark Night Plus).
I've googled on this one and I think I must be the only person who things the waistband on the Sweet Legs Leggings Plus is too tight. The rest of the leggings fit great with lots of room to spare, but that waist band! Argh! Because of this, I never wear them. I have 3 pairs, and I've worn 1 pair once.
I pulled my least favourite pair out of the drawer, grabbed my seam ripper, and seam ripped the stitches that held down the waistband (cover stitched originally), then seam ripped the stitches that attached the waistband to the pants (serged originally).
I had to be really careful not to catch any of the fabric, but it went well, and I ended up with the elastic coming out intact enough I'll be able to reuse it for some pj pants for one of the kids.
I then measured the length of elastic I wanted to use to be comfortable. I just used the 1" braided elastic you can get from Walmart. I think it was less than $2 for the length and I can easily get 2 waistbands for myself plus at least 1 for the kids out of one package. I want to try cotton swimwear elastic, but I'd have to order that online, so I haven't yet.
I used to be an hourglass before I had the kids, but I'm definitely more of an apple now. Needless to say, I was weary of using elastic in my waistband at all, but because I'm not an hourglass, my best shot of having my pants stay up is with some sort of elastic (or so I've heard). It's about as loose as it could possibly be too, so it's pretty comfy.
I cut the elastic to length, and sewed it up using my regular sewing machine and a zigzag stitch. I placed a scrap square of fabric under the 2 ends and sewed over that too (trimming the sides off after sewing). It worked well! My zigzag settings for sewing the elastic together were 2.25mm width and 0.5mm length. I'm a chicken, so I zigzagged over 2 times and back-stitched on each end.
I quartered the elastic and the pants (separately, with ball point pins) and pinned them together at those quarter marks (on the wrong side / inside of the leggings).
Using my 3 thread narrow stitch on my serger (I love LOVE LOVE my Juki 654DE), I attached the edge of the elastic to the inside edge of the pants, and was super careful not to nick the elastic. I contemplated sewing with my blade disengaged, but wanted to get any pesky bits of leggings fabric that was sticking past the elastic, so I left it engaged.
I then folded the elastic down one time to hide it, but still leave that serged edge exposed. I contemplated rolling it down once more, but tried it on like that and it fit better just being folded down once. After folding the elastic down, I quartered it again and pinned those quarters down. I love pins.
I went back to my regular sewing machine and used my 3 step zigzag (5mm width, 1mm length) to sew down the elastic and hide it from sight. I sewed with the wrong side facing me and kept the zigzag as close to the serged edge as I could.
I think it came out pretty well! The leggings are now super comfy and I have pulled out my other two pairs to seam rip tonight. I took note of the length of waist elastic I found comfy and will easily be able to replicate this alteration on future pairs.
I'm particularly excited because I'm in time to order a few pairs of holiday Sweet Legs and have them altered in time for winter!
I've also drafted a leggings pattern from the sewhere.com Made to Measure leggings class and I'm excited to start cutting and sewing some super custom leggings for myself in the next little while.