Start planning to wire the switches from all sixteen buttons to the Raspberry Pi Pico. The wires will be soldered to the bottom of the PCB.
Place the Raspberry Pi Pico over the opening in the top cover to reference how long the wires will need to be in order to reach the PCB.
Wires for Switches
Each wire will most likely have a different length of wire. To help keep them organized, use heat shrink tubing, tape or similar to keep sets of wire bundled together.
Here I've created four sets of wires, each set having four wires.