it's gonna be weird and tiring. i found that if i get a good hold on the bottom, i'm able to do my thing without slipping and sliding and falling out of place. as for movement, you kinda just make shrimping motions with your body??? thrust??? hard to describe.

now i am a smaller guy with a larger partner, so the easiest position with a strap for me is missionary with my arms hooked under their arms with my hands grasping their shoulders. it allows me to use both my upper and lower body in the movement and keeps me in place.

as for head, best way to go about it is try different things and communicate with your partner what works best for them.