I've been wanting to do this tutorial for a while now. Instead of just putting it in my Adorabubble Britches Pattern, I thought I'd share it with everyone. This technique can be applied to most anything that needs and elastic casing. Best part, no sewing!
Start with a provisional cast on, second video from the bottom. Instead of using a piece of waste yarn, I like to use another circular needle (same size or smaller).
Work back and forth (do not join in a round yet) until piece measures just slightly longer than the elastic you plan on using. End having just worked a wrong side row, right side should be facing you.
Purl the next row. This will make a ridge that makes it easier to fold and keeps things neat.
When you get to the end of this purl row, you'll join your piece in a round knitting the next round.
Knit even in a round until the ends match up when you fold at the purl ridge.
Fold at the purl ridge and line up the provisional cast on edge to the working edge. You will now knit the cast on edge to the working edge.
Insert the right needle into the front stitch (working edge) and into the back stitch (cast on edge), draw loop through and pull both stitches off. You will now have 1 stitch on the right needle.
Just a note, every other stitch on the provisional cast on edge will be twisted. On those stitches you can just insert the needle through the back loop or if it bothers you, before knitting the edges together, you can rearrange the cast on edge so the stitches are all mounted correctly. Honestly though, if you just knit them as is, no one will know. It's on the inside edge anyway.
Voila! How nice does that look?!
At this point, if you patterns requires it, you can switch to larger needles and do your increase row for the hips. Once the whole piece is finished, measure out a length of elastic about 1" longer than the waist. Pull through the hole in the back, secure and sew shut. Ok, I guess it isn't completely no sew but mostly.
This also works for enclosed drawstrings as well.