You have to 3D print the parts and have the boards for the keyboards ready

start with soldering the diodes to the keyboards, then the tactile button switches

Connect the two PCF8574 to the keyboards (also join together the row pins) and set the address pins differently.

Go on with the power compartment - solder together the switch, LiPo, LiPo charger and boost converter

basically follow this tutorial to get the keyboard driver stuff ready