The keyboard is printed in two halves which are symmetrical so you can mirror the included .STL in your slicer to print the other side.
There are two 3 mm holes on the sides, you'll need something (like a nail or 3D printed part) to create a friction fit between the two halves to stop them rotating.
The base will solidify the two halves together, I used acrylic which I cut to size with a hack saw and drilled some holes which line up with the eight holes of the keyboard. The acrylic is secured to the two 3D printed parts using some heat set inserts and M3 screws. I designed the holes so you could screw straight into the 3D part but I'd strongly recommend getting inserts because taking the base on and off repeatedly will eventually destroy the threads of the 3D printed part. If you insist on not using heat set inserts at least start with shorter length screws and work your way up to longer ones as the thread weakens.
To add some tilt you can put a couple nuts on the rear screws. The author had to add some small washers to the front left and right screws to stop it wobbling.