There's a few different ways, mine tend to be trial and error and turn out interestingly different each time. rolleyes
Hopefully someone who's a bit more accomplished at sewing will be along soon to give you advice. If it runs the whole length of the cage you will need to add a few emergency exit holes to avoid scuffles. I'd advise you putting a few shorter ones in a row, that way if one gets soggy and the others are dry you don't have to wash the whole lot for the sake of one wet spot, they'll also be easier to shake out.