The amount of water depends on your brush, I use a cheap silvertip brush, and water that is close to boiling. soak the brush, as well as your mug scuttle, whatever. when you are ready to lather, pour the water out of your mug, and let all of the water run off of your brush. if there's a little left, that's okay, I usually give the brush a little flick into the mug.
swirl the brush in the soap until it is well loaded, and starts to form a lather, then move to your mug.
do not lift your brush, keep it close to the bottom of the mug, swirling and pumping the lather until it is reasonably well developed, and then move to your face.
It should be good and hot, and you can continue to build lather on your face as needed.
Pretty simple, but gives good results. and the lather should still be warm for a second pass.

Just remember you can always add water, it's harder to add soap.