I've tried using the "Dip a cup and pour" method. Never used a cup like that before camping without showers. It only took once before I came up with an improved idea.
If you can purchase a "Weed Sprayer", (Try to find one with a short nozzle. The shorter the nozzle the easier to maneuver the wand around and under your body - especially for rinsing the hair).
We then filled the canister about two-thirds with warm water, ADDED the pump handle, THEN pumped for pressure. Walla, a warm shower. When camping, this idea worked great for my wife and me.
P.S. It's a good idea to use a new sprayer. Good luck