Power down the circuit you're measuring.

Choose the point where you want to measure the current. You'll need to break the circuit at that point. This may mean snipping a wire, disconnecting a connector, or removing a jumper wire.

Connect the multimeter in-line with the circuit by touching the probes to the ends of the broken circuit. If possible, use alligator clips to hold the probes to the ends.

Power on the circuit and make your measurement.