Dyeing fabric is easy...more folks ought to try it...gives you a whole new palette. (my avatar is a silk scarf done with acid dye)
For the denim, you'll need fiber-reactive dyes. Dharma Trading or Pro Dye are both great companies. You'll need soda ash, salt and urea. (all sold by these vendors)
Mix your dye..(wear a dust mask..it is toxic) I use warm water and mix in fruit jars. About a teaspoon in 3/4 pint of warm water. Mix well...add 1/2 teaspoon of salt (non iodized) and 1/2 teaspoon of urea. Mix till dissolved.
Soak your denim pieces in soda ash water. I use 1/2 cup to a half sink full of warm water. Pile the denim in and let it soak for about 20 minutes. You can wring the denim out and use it damp, or let it dry...either way is fine...reuse the soda ash water as long as you can...if it gets dingy looking, I generally pull the plug.
Cram the denim into a container...plastic is ok...even ziplocks. Pour dye in and let it sit overnight. Rinse well...I generally run them through 3 wash cycles. Hot wash/cold rinse. First time, nothing in the water. Second time, a cup of white vinegar. Third time a small drizzle of Dawn dishwashing liquid. That's all there is to it! There's a little learning curve, but you'll get the hang of it quickly.