Should be easy to assemble, there's pegs to help you align the pieces.
You'll need to print 6 large pegs and 2 small pegs, the attachment piece needs the small pegs because there wasn't enough room for the normal sized ones.
The bottom part attaches to the top with magnets, there's slots for 4x 6mm (diameter) 3mm (height) magnets and 4x 10mm (diameter) 3mm (height) magnets on each side. I only ended up using half the slots because the magnets I used were powerful enough, but if you want it more secure you can fill all the slots.
The slot for the hand is 12x12cm at the back, and about 11x8cm at the front. It's quite loose on my arm, but it should accommodate most hand sizes.
There's also slots for 1 inch wide webbing straps to attach the base piece to your arm, so you can swing it around freely!