Read the operating manual of the frequency counter to be used. Some counters provide a variety of information including frequency ratio, time interval, period and pulse width. More advanced models also give additional readings, such as time interval average and time interval delay.
Follow the directions given in the operating manual to hook the frequency counter to the source of the frequency.
Read the number that appears in the digital display. This is the frequency of that source. Adjust the setting on the machine to find additional information such as frequency ratio or time interval.