We will be turning the boost on/off via the ENABLE pin, so the switch does not have to carry any power, it is only signalling to the boost converter what to do. This means the switch can be small. Use any switch you like, if it is 0.1" spacing thats ideal. If your switch only has two pins instead of three, tie one pin to GND and the other pin to EN - when the switch is closed, the power will turn off
The switch will go into the breakout header, and attach to VBAT EN and GND
The switch is symmetric so as long as those three pins are attached to the switch you are good to go
For mechanical strength, we soldered in all three pins. However, this only works for "break-before-make" type swtches! If you are not sure that your switch is "break-before-make" or if your switch has only two pins, solder in ONLY the EN and GND pin, skipping the VBAT pin!