Fat Dreadlocks: Thick dreadlocks should be backcombed. This way you can chunk them up before crocheting.
I am a huge advocate of crocheting dreadlocks. I would say that unless you are going for the natural look, a crochet needle is a must. You can get them at any craft store for about $3. We have about 8 crochet needles that we share in the house. They get lost easily, so it might be a good idea to invest in a few. I prefer the smaller sizes, but anything under 1.5mm will work just fine.
Long thin dreadlocks: Crochet only! If you don't want to lose hair length in the dreading process, don't backcomb. Your hair may loose a few centimeters, but will generally be as long as your original hair. This can be done by twisting the dreadlock. Pinch the dread between your thumb and pointer finger. Push the tip of the crochet needle between your fingers, the hook facing your thumb. Push the needle in and out, between your fingers, through the dread. Everyone has their own way of doing it, so after some practice, you'll probably start figuring how what works for you.