After you have built IO-Board, Mainboard and RF Board, the Sandwich needs to be assembled.
This is pretty straightforward:
Just stack the layers on Top of each other.
Start with Top Layer (IO Board). Use 12mm Spacers (M3 Female – Female) and mount them with 4x M3 Screws from Top. (3x M3 Screws in Case of LCD IO Board)
Then, stack that on Top of the Mainboard, and secure it with 10mm Spacers (M3 – Male / Female)