I make belts for Ava all the time. Basically, you just need to go to the fabric store, buy some 1 inch elastic, and sew velcro to it. Be careful to use a thick needle, or it will break.
I don't bother to cover it with fabric, she doesn't like the bunching. This way the belt lies flush on her and the velcro makes it adjustable. If you can find it, get velcro without glue on the back. It makes it very difficult to sew through the glue, though I have a hard time finding it.
Oh, and when you cut the elastic band, if you have a gas stove, brush it quickly over the flame to melt the end. This will keep it from fraying.
Mom to Ava
Animas PING 2008
Dexcom (the other love of my life)
At the height of laughter, the universe is flung into a kaleidoscope of new possibilities.