I’ve just invested in a peg loom myself, but if you would like to make one instead, then here is a good tutorial using dowel, and this one is an even cheaper version using scrap wood and nails.
The second tutorial shows you how to weave using t-shirt yarn, but you could use plastic bag yarn or regular yarn instead of course, and you could make scarfs, table runners, wall art, beach mats. . . loads of stuff!
Go here for my previous post on yarn.