When I do the elastic I tack down one end by doing a zig zag back & forth. Then I switch my stitch to a triple zigzag & pull the elastic nice & tight as I sew it down. Then go back to a normal zig & tack down the end.
I turn & top stich my nappies so once I do that I then go around & do my top stitch & when I get to the leg elastic I move the nappy so my stich (which I put back to the widest tripple zigzag) and go along the side, but as you do this make sure your not catching any of the elastic or seam edge (thats inside the nappy) other wise it will stuff you up & you'll need to unpick it.
I will do a photo of each stage of how I do mine later tonight. Been menaing to do that for ages.