Might be doing it right, not sure...
Here is what I have done lately, it seems to work for me. Still experimenting.
Large mug, smal soap cake. Melted it in the microwave for about 15 seconds, now covers the entire bottom of the mug.
Got the water absolutely steaming hot, placed about 1/4 tp 1/2 inch of the hot water to cover the (now melted) cake completely. Insert brush and leave alone.
Filled the sink with enough hot water to place mug in, not so much that it goes over the lip of the mug.
Left the entire affair alone for about 3-5 minutes.
Emptied water in sink and poured water out of mug.
Shake off brush, swirl, push, whatever vigorously until it looks kind of like meringue topping.
Apply and enjoy.