You'll be testing a live circuit. Be very careful not to touch any exposed circuitry. If there's a significant chance of exposure, cover the live circuit with insulating material or wear protective equipment such as insulating gloves.

Be careful as you position your probes, making sure they don't accidentally short anything. If possible, power off the circuit while you set up.

Touch the probes across the AC source and make a measurement.

If you're measuring something that's electrically noisy (such as a compressor motor), you can toggle the Variable Frequency Controller (VFC) function to filter high frequency noise out. This will reduce the precision of the measurement.

To toggle the VCF mode on/off while you're measuring AC frequency, hold the blue Select/VFC button for two seconds.