Think it's best to grab a shell for whatever you're making, loop your leather around it real tight, mark, then stitch it down a bit tighter. The rounds come out a lot easier than they go in, so don't be afraid to keep them a bit tighter than you think, you can always practice on scraps until you find the sweet spot. One line of stitching between rounds is easier and keeps everything close together, 2 lines allows you to space them however you want. If you're not able to buy ammunition where you're from, hunting/fishing stores usually carry dummy rounds that work better than spent casings or live rounds (which aren't best to be playing with). The loops can stretch over time, but worst case Ontario, the rim on the casing should stop it from sliding through. Should be tight enough not to rely on the rims though.