Mark the spot where the sub-panel is to be located. Locate the panel as close as possible to the main service panel if it's to be used to extend the breaker capacity of the main service panel or in the area of high load density if it's to be used as a load center panel.
Hold the panel in place against the wall using the level to make sure the panel is level and vertical before marking the holes for mounting. Drill the holes for the mounting hardware using the drill/driver and the appropriate size drill bit.
Secure the sub-panel in place using the appropriate mounting hardware from the hardware kit.
Remove the cover of the main service panel and remove a metal "knockout" from the side of the main service panel box. A "knockout" is a solid metal disk that's secured in place at one or two points. To remove it, use a hammer and screwdriver to push it horizontal and then grasp it with the Lineman's pliers and pivot it up and down until it snaps free. Install a cable connector in the knockout's hole.
Remove a knockout from the top of the sub-panel box and install the other cable connector.
Route the sub-feeder cable from the main service panel to the sub-panel. Cut it long enough so that once it's inserted in the main service panel and sub-panel, the cables individual conductors will reach the bottom of the panel boxes. Insert the cable through the cable connectors and secure in place by tightening the two screws on each of the cable connectors snugly down against the cable's jacket. Use caution when inserting the cable into the main service panel because parts of the service panel will still be "hot" even after you turn off the main service breaker.
Remove the outer jacket of the sub-feeder cable leaving ½ inch of the jacket intact where it exits the cable connector. Be careful to not damage the insulation on the individually insulated conductors. Any damage to the insulation on these conductors will require you to replace the sub-feeder cable run with a new run of cable.
Cut the red and black conductors in the sub-panel to the length necessary to reach the Buss Bar lugs at the top of the panel. Cut them long enough so that they aren't under constant tension. Remove 1 inch of insulation from their ends and secure the stripped ends under the compression screws. It doesn't matter which wire is connected to which lug.
Route the white neutral wire and the bare grounding wire down to the "Neutral" and "Grounding" bars respectively. Strip their ends and secure them under the main compression screws on the neutral and grounding bars.
Install the 2-pole, 240-Volt circuit breaker in the main service panel. Depending on the type of panel that you are working with, this breaker may simply plug in or it may be secured to the panel's Buss Bars with screws. If it's screwed down to the Buss Bars, make sure that the main breaker is turned off before attempting to screw them down.
Connect the red conductor and black conductor to the brass screws on the new circuit breaker. Connect the neutral conductor and the grounding conductor to the panel's neutral and grounding bars.
Replace the service panel cover. With the new breaker in the off position, turn the main disconnect circuit breaker back on. You will leave the new breaker in the off position until all the branch circuits to be fed by the sub-panel are connected to the sub-panel.