Shower. I rinse it out, put on the post color conditioner, wash the rest of me and then rinse the conditioner out. I do have a shower head that I can take down and use as a handheld if I choose to, but I don’t seem to need that when rinsing it out. I do keep a towel hanging where I can reach it if I need to dry my face after the initial rinse, which is pretty normal.
I have a very small bathroom sink, and would get dye all over the place if I tried to do it in the sink.