What is the most common form of Femdom practice? is it slaves getting tortured, or slaves getting humiliated, degraded as lowly as possible? (such as become foot stool, foot licker, door mat, dog, spitt-oon, even toilet for the Female Dominants?

Usual is being used as a footstool. Foot kissing is one of the most obligatory things.
Getting kicked is quite common as well as being used as a door mat.
Facesitting while the ladies sit with their full weight just likeon a cushion is also a usual practice.
And of course a slave is often punished by a riding crop, whip or something similar.
Making the slave to clean the toilet of the ladies with his tongue is not unusual and riding on the back of the slave is a common pleasure as well.