The easiest way if you are just doing a few(or doz or so) is to go to JoAnn's or Michaels or somethign and get the tye-dye kit, or individual colors. The directions and everything you need should be included, and they are pretty inexpensive.
I think the brands are usuall, rainbow rock, dylon, or something similar. I would stay away from rit, it is not a great dye.
Have fun. I did a few for one of my kiddos, and it wasn't too hard.
The other option is to go to
www.dharmatrading.com and get the supplies from there. They have lots to read about on the subject of dying and they have blanks too, so you can even get tiny t-shirts and stuff to dye to match the prefolds.