Solder data connections to Pro Trinket according to the circuit diagram.
Twist and tin the two power wires together and slide on a piece of heat shrink tubing. Solder to BUS on Pro Trinket, then silde and shrink the heat shrink over any remaining exposed wire.
Double check there are no stray strands of wire, or these could short out and possibly causing damage!
Repeat with the ground wires, but add one more wire to the bundle that will go to the membrane keypad's ground wire.
Plug your membrane keypad into some female jumper wires and lay out the assembly to gauge the approriate length to cut the jumpers.
Solder the jumpers to digital inputs and ground as described in the circuit diagram. Use heat shrink tubing where applicable.
Test out your circuit to make sure everything works as expected before proceeding!