Now that your T-shirt yarn is all cut out, it's simply a matter of rolling the yarn into a ball and you can use it just as you would any other yarn.
Before you start rolling, though, you'll probably want to encourage the yarn to roll up on itself, leaving fewer of the raw edges exposed.
To do that, take an length of the yarn at a time and give it a hard tug between your hands. You can also roll it a bit between your fingers as you go to help the raw edges roll to the inside.
Even after doing this the edges might not be completely perfect, but that's part of the fun.
