I picked up one of these with the longer beam and this 14" knife 2nd hand.

But I am not sure how to mount it up. Like at what height etc. to mount the bottom of the beam.

I'm a completely average 5'9" tall.

That's going to depend on your preference, do you want a horizontal or a downward angle? I only know the down angle and prefer the beam to be a few inches below my sternum with maybe a 45 degree angle. I would start with the angle and since it is adjustable, I would see what like or don't like and adjust from there.

Most of the guys I know like the base mounted 6-8" below their belt line and the angled up to their belt line. Not 45 degrees but closer to 33 degrees. Less bending and use their body weight to hold the pelts in place against the nose of the board. It's all in what you get used to so I'm told. I don't flesh coons or beaver and use a flat beam for coyotes, mink, rats, fox, etc... you know the easy stuff.

Thanks guys! I'll experiment a bit with it but this gives me some good starting pointers.

Do you have a sawhorse? If so, just put the sawhorse under the beam and slid it back and forth until you have the right angle.
Then drive a nail through the edge of the beam into the sawhorse. Do both edges. If you have a stud wall just screw the mounting bracket to the studs.
Another thing to think about is using a gunny sack to tie around your waist to keep the hide in place as your working on it. What I have done was use an old set of waders that couldn't be repaired. Just cut off the legs above the knees. It keeps you clean and even keeps you warm, but it keeps the hide in place, and it gives your mid-section a little cushion.