Inspect the breaker box and familiarize yourself with the mounting holes that are located on its sides. This is how you will mount the box to the wall studs, and knowing in advance where all the holes are will help you quickly and easily install the box without fumbling around and looking for things while you are trying to hold the box in place and screw it up at the same time.
Hold the box up between two wall studs at a height at which you are comfortable working. The exact height is not important, although most breaker boxes are installed around shoulder height for ease of access when the electrician is wiring the breakers. Plus, this height makes it easy should you need to replace breakers later on in the life of the house.
Mount the box to the wall studs on either side by using the screws provided in the box kit. They will go through the inside of the box via the pre-drilled holes, allowing you to simply screw directly into the wood framing through the holes. Ensure all of the screws are snug and tight and use all of the screws provided in the kit.