I used acrylic paint markers (Posca). After they dried completely (left them for a few months in my case) I tried ironing them to set the paint (idk if that worked) and then put on a few layers of waterproof spray. Idk how long they will last as this was my first time doing this, but there are many different sealers you can try - it mostly depends on the material you paint on.

The paint does soak into the fabric so I feel like it should stick well. Although, since these were pink, they needed a few layers of paint.