Install a circuit breaker by turning off the power, running the wires to the breaker panel, attaching the wires to the new breaker, and testing the circuit. Be extremely careful during the whole process and ensure all connections are properly insulated before switching the power back on.
Turn off all power switches before performing any electrical work. Place a note on the power switch so no one turns the switch on while you are working. Use a voltage tester to confirm the panel is de-energized before installing the breaker. Hold one probe against the neutral bus bar while touching the other probe to both setscrews on a double-pole breaker. The wires going to the main breaker are always energized; do not touch them.
Before buying a circuit breaker, calculate how much load the new circuit has and compare it to the rating on the breaker. If your new circuit is over 1,920 watts, you need two circuit breakers.
Place the new circuit breaker in the Off position before installation. Remove the knockouts from an unused section of the circuit breaker. Rip the sheathing of the new circuit's cable, exposing the wires, and run them through the side of the panel. Strip a half-inch from each wire, attach the white wire to the neutral bar and attach the black wire to the breaker. Make sure all set screws are tightened and all excess bare wire is trimmed.
Secure the breaker into the slot. Switch off all breakers before turning the power on. Turn each circuit on one at a time and test your new circuit.