Turn off the main breaker and verify the power is off with a voltage tester
Check local electrical codes and obtain required permits
Choose the subpanel location close to the main panel and with proper clearance
Mount the subpanel securely to the wall
Install the feeder conduit or cable from the main panel to the subpanel location
Run the correct size feeder conductors and grounding conductor
Install a double-pole breaker in the main panel sized for the feeder
Connect the feeder hot wires to the breaker
Connect the feeder neutral to the main panel neutral bar
Connect the feeder ground to the main panel ground bar
In the subpanel, keep neutral and ground bars separate
Install an isolated neutral bar if required by the subpanel
Connect the feeder hot wires to the subpanel main lugs or main breaker
Connect the feeder neutral to the isolated neutral bar
Connect the feeder ground to the ground bar
Install branch circuit breakers in the subpanel
Connect branch circuit hot wires to the breakers
Connect branch circuit neutrals to the neutral bar
Connect branch circuit grounds to the ground bar
Tighten all terminals to manufacturer torque specifications
Label all breakers and circuits
Replace all covers and dead fronts
Restore power and test the subpanel circuits
Verify proper voltage and breaker operation with a meter
Have the installation inspected if required by local code