At first, this was going to be a 60% keyboard, but after a lot of design decisions and switch comparisons, [flurpyflurples] settled on a certain micro switch spaced at 7.3mm for a 40% layout.

Edit the constant variables to match your keyboard switch matrix. The important things to change are the inputs[] and outputs[], and keymaps. A Pro-Micro doesn't have to be used, the Arduino board ...