Step 1: Wire Wrap The Push Buttons
For my build, I did not use 1.25cm spacing throughout. I started with .75cm and went up to 2cm. This resulted in more uneven keys I think, so in the future I'll probably just use 1.25cm or 2cm consistently.
Test your push buttons so you know which two leads are the same, and using the needle nose pliers, form two small wire loops a distance apart as wide as your push button leads, around 5 mm.
Straighten the push button legs, and place them through the holes then fold them out to hold the push button in place. Slide the insulation over (you may need to use some strength to do that) and form two more loops for the next push button. Attach six push buttons to the first six lengths of wire.
Using the remaining six lengths of wire, once again cut slices in the insulation about 1.25 cm apart. Then carefully link the rows of buttons together. Flip the entire grid over, and carefully solder the backs of all the rows and columns in place.