Novice trying to create a countdown timer

Hi. I am a complete novice on using app inventor and so have gone through many tutorials and beginner books but am struggling on making a countdown timer. My Mrs has asked me if I can add a countdown timer to a little workout app I was making for her so that she can choose her workout time and rest period. Then when she starts it a beep will sound at the end of work out and then the rest period and it loops until she stops it. I’ve been up all night trying to look up bits on this forum to no avail and would really appreciate any help anyone could give. A big thank you in advance.

Simple Countdown

Maybe something like this:


Thank you very much for this. I will give this ago.