I start by setting then metronome to click at a rate of 12 beats per minute which will mean it times one beat every 5 seconds.
To help with this I use the Metronome Beats app on my Android phone. When I am doing interval strength training (for example doing arm exercises with weights), although I may start with an even 5 seconds lift/muscle contraction, 5 seconds relax, by the end of my set I have lost this timing completely as I just try to get it finished! I also constantly lose track of how many repeats I have done. Are you following an exercise workout which requires the use of slow/timed repetitions? Do you struggle with maintaining an even tempo as you contract and relax your muscles or forget how many repetitions you have done? A metronome such as Metronome Beats can help you with this timing, leaving your mind free to concentrate on doing your exercises properly.