On your four. Ass up facing your Sir.
Invite your sir to use your back as a foot stool to rest his foot.
Your mouth should be filled with your own underwear
Your head must be remained to be looking up to the front and hold still as you are just a furniture to your Sir.
With the permission of your Sir, you may now masturbate till you cum.
After you cum, remain your foot stool position till he is pleased.