You brew coffee, pour it into your large container. Then add your fabric. I've never heard of doing it in the bathtub, but I suppose you could. Tea dying is much the same, but you use boiling water and tea in bags or sieves. I couldn't do either of these in my bathtub - it would permanently stain.
The only problem I have had is getting the tea dye (or coffee dye) to remain wash after wash. It just doesn't stay set. I guarantee you, vinegar doesn't do it. I haven't tried it with Retayne.