I use about a tablespoon of clothes detergent. Sounds tiny, I know, but it's a relatively small space and (more importantly) it's a pressure washer. The pressure makes a huge difference. Just make sure to put the cap on tightly, or it will shoot off after the first two turns (possibly popping you in the face... yeah, there's that much pressure built up by the turning). If you use too much soap, rinsing takes forever, so use it sparingly! You'll be amazed at the amount of dirt in the water!
For the past month, I've been without a traditional washer & dryer (ex got-em) and use my little wonder wash exclusively. A week's worth of clothes (shirts, undies, socks & one pair of pants) will fit inside. I'll spin the tub for two minutes... 60 cranks in one direction, then 60 in the opposite direction. Dump the water (from the top for speed), add fresh water to rinse and repeat the spin cycle twice to make sure the soap is out. Hang to dry in the bathtub.
Cheap, easy, and washes faster than a traditional electric model.
The next thing i want to buy or make (probably make) is a clothes wringer. My theory is, if I can wring all the dirty, soapy water from the clothes before adding the rinse water, I'll only have to rinse once, and thus save water.
It's such a great little machine. You won't be disappointed.