Ive done it, 2 different ways, with similiar results.
One way is to use a cold water dye (Dylon do them), you basically put dye plus water plus salt into big bucket and put jeans in, stir every so often (good instructions in dye pack) - and then do the rinsing etc yourself.
Other way os to get Dylon Machine dye - this you put into washing machine with your jeans, and let the cycle run (again instructions given), and you dont have to worry about stirring or rinsing. But you do need to run the washing machine with nothing a couple of times after (or with small amount of bleach) to make sure all dye is gone.
My preference is cold water bucket method, I just feel you have more control and less risk of wrecking the washing machine!!!
A dry cleaners, good supermarket or maybe somewhere like Hickeys should have range of Dylon dyes.