My first diaper would have been useable, but the butt was WAY too narrow.
After that, they've all been useable, even if they're ugly.
For zig-zagging over the edge, I highly recommend an overedge foot. It just makes it SO easy to get it perfect every time.
For elastic - don't measure. Just tack it down on one end, then stretch it as far as you can (if you're using polybraid or swimsuit elastic - for Lastin, stretch until the stretch "changes" - don't go beyond that point), then sew either a straight stitch, a zig-zag, or a 3-step zig-zag. I do the latter. When you get to the end point, tack that down, THEN cut your elastic.
I know it's hard to go from the apparel world where you're told how much elastic you need, but with diapers, you just leave it on the long piece and sew it on, cutting after you've sewed it on. Much quicker and easier that way, and you don't have to worry about running out.
If you're stretching it enough (using the rules above), it will work fine.
You might try doing the turn & topstitch method for the VB
. It has instructions in there for doing that. But I can't say enough about the overedge foot. I can't sew a straight line to save my life, but I've made an entire VB
fitted by zig-zagging with the overedge foot.