Switch off the power. At the electrical service panel, locate and switch off the main circuit breaker that provides power to the main panel circuit breakers. Be sure to check for the absence of electrical power with your circuit tester to make sure that no electricity is flowing into the two main hot bus bars that run down the center of the electrical panel.
Mount the subpanel according to the manufacturer's specifications at around five feet above ground level. You should anchor the subpanel with the screws provided either in between studs or on a piece of cut plywood that is screwed in between studs.
Insert the properly gauged electrical feeder cable into the subpanel by removing any one of the circular, disk shaped metal "knock-outs" from the side of the subpanel. From inside the subpanel, strip the sheath of the feeder cable to about half an inch from the subpanel's knock-out while leaving enough loose cable length to connect it to the various subpanel bus bars. Connect the red wire into one of the subpanel's hot bus bars and the black wire into the other hot bus bar. Connect the white electrical cable to the neutral bus bar and the bare ground wire to the grounding bus bar. All connections to the bus bars are secured in place via the primary bus bar screws. Please note that these connections are diagrammed according to the subpanel's manufacturer's specifications and should be noted in the subpanel's installation sheet. Use cable clamps as necessary to ensure that the cables that enter the main panel or the subpanel do not get damaged by the sharp knock-out holes.
Secure the cables safely. At the main service panel, insert and tighten the red and black feeder cables into a 220-volt double pole breaker. This breaker will then need be popped into an available hot bus bar slot. The amperage rating on both the cable and the breaker will depend on the amount of amperage you will want available in the subpanel. Connect the white neutral and bare ground cables to the neutral and ground bus bars. Be sure to have enough feeder cable slack in the panel to provide you the cable length you will need to safely work with. From inside the electrical panel, the main feeder cable sheath should also be stripped to about half an inch from the panels' inside wall. All cable ends should be stripped to about half an inch from the cable ends to ensure they are properly screwed in to either the bus bars or the circuit breakers themselves.
Check your work. Before switching your main circuit breaker back on, ensure your cables are tightened, your circuit breakers set correctly and that your cables are free from damage.