Remove the controller board and matrix from within the number pad and take a look at how things are laid out.

Rubber dome key switches work by pressing contacts on each layer of the matrix together. The goal in this build is to replicate this same matrix but with mechanical switches.

Separate the three pieces of the matrix and discard the blank, middle piece.

Each dot on the matrix represents a switch, on a number pad it's pretty easy to keep track of what's what.

I went through and labelled which row/column went to which pin on the controller board. This can be done just by following the traces back to the plugs on the board and comparing them.