Hiya!
You could use fabric paint, or acryllics with an acryllic fabric medium (I've used both, both work well - I noticed the medium mix started to crack some though)
You could DIY silk-screen it (that requires putting together a silk screening kit...maybe Youtube has some cheaper alternatives to show you...) and it would require stenciling, but the's a very long lasting and professional way to go.
Or depending on how you want to design it, you could use the printable iron on transfers...
hope it helps some.
** if you do use paint, make sure you put something between the layers to prevent bleeding through.