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 connecting 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.

The polarity of the probes somewhat matters. The multimeter will measure current flowing into the red probe and out the black probe. If you reverse the probes, it will show a negative value on the multimeter, but it won't damage the meter.

Confirm that the probes are only touching what they should and nothing else.

Make sure the circuit isn't accidentally touching you or anything conductive that may short it.

Power on the circuit and make your measurement.