Shower always. If DCing with AO GBP, I apply it to dry hair 15 minutes before I take my shower. Jump in the shower, shampoo, condition. Put on cap while I wash my body. Rinse body, rinse hair, towel dry; spritz with ACV solution.
If using Emergencee or for the mid-week wash, I stand on the side of the tub (I have one of those showers that is also a tub; shower has a hose and shower head), and use shower head to wet hair, use the switch to turn showerhead off and lay it on the faucet, shampoo hair over shower. Rinse. Emergencee, put on cap. Go enjoy a cup of coffee or whatever. Return ten minutes later. Shampoo with Therappe. Rinse. Condition. Wait a few minutes (I usually fit in something during the waits like a quick email or post). Rinse. Mix ACV solution in a basin. Dunk head, massage scalp. Done. I suppose doing it this way is like washing over the sink...but without a showerhead I think it'd feel really clumsy. Plus over the tub I don't feel confined. I think I would be over the sink.
If I am not DCing, (both previous paragraphs involved DCing), then I do it in the shower. There's no need to get out...because I shampoo my hair first, apply conditioner while I finish my shower then rinse conditioner out. ACV rinse or spritz to finish my wash (I usually have the basin in the tub with me so all I need to do is add water, the ACV (I keep a small bottle in the b/r, and EOs also in the b/r).