Step 3: Battery
The terminals on the battery are 1/4" male spade connectors. These are easy to work with, by crimping spade connectors onto wires and then just pushing them onto the battery connector terminals.
The battery is marked with the Positive in Red and the Negative in black, and has a plastic protector around the Positive Terminal to help reduce accidental short-circuits.
Put the battery into one half of the power supply case to ensure that it fits. You may want to use a pencil or marker to outline it so you know where the battery lines up without it being in the case.