Set your multimeter to the highest voltmeter setting (e.g. 600V on the one shown) and ensure the probes are connected to the correct terminals.

Connect the red probe to the solder on the red wire and the black probe to the solder on the black wire.

WARNING: Be careful not to touch the two probes together while doing this. It would short-circuit the capacitor and dump a large amount of charge very quickly. Also be careful not to touch the metal on the probes.

Look at the meter reading. It could start in the >300V range. If you don't see a reading, make sure the probes are making a good connection. If they are, and you still don't see a reading, drop the voltmeter setting a couple of notches (e.g. to 20V) and try again. The capacitors may already be (mostly) discharged.

You should see the voltage dropping as the capacitor discharges through the multimeter.

Continue to discharge until the voltage reduces to a significantly lower level (e.g. <20V), then switch to a more sensitive voltmeter setting (e.g. 20V) to maintain accuracy and continue discharging.

Discharge speed will slow down the lower the voltage gets. Once it slows to a crawl at a low voltage (<10V), you can intentionally short-circuit the capacitor with the probes to quickly discharge the remaining charge.

Verify the capacitors are fully discharged using the multimeter at a more sensitive setting (e.g. 2V). The reading should be at or close to 0V. Note: If the reading starts increasing it's because the multimeter is starting to charge the capacitors. Short-circuit them one last time and turn off the multimeter.