What fabric? Fleece, wool, hemp, bamboo? Need one NOW!!!
Here's a picture of one of my boys in one.
Here's how I do it.
Measure your baby's waist. Cut a piece of 1inch elastic about that long. It has to stretch over a diaper, but if it's too loose it won't hold the diaper in place. 17 inches was perfect for my boys between about 3 and 10 months. Sew the ends together to make a loop. Cut a piece of fleece a little longer than your elastic and 3 times as wide. Center the fleece behind the elastic and wrap the fleece around the elastic. so you have one layer behind and 2 layers in front. Sew down the middle with a zig zag (my machine won't, so I stretch as I sew) and sew down your open end.
Skycheattraffic, your toddler probably won't ever need a bigger size. Our 11 month old just sized up because he is a mega huge baby. He was a 9 pounder and wore a 17 inch belt from 5 weeks (when I made his first one). Our 5 month old was only 7 pounds so we started him in a smaller size. When he grew into the bigger belts we just made new, bigger belts for our older baby. The fleece loses a lot of its fluffy cushiness if its used 24/7, so new ones for size changes seem reasonable. The big guy can still fit the 17 inch belts with a thin diaper and he's 25.5 pounds so the elastic really stretches.