I just used my phone’s stopwatch. For the first 30 seconds, I did the first exercise, and when it hit 30, I switched to the second exercise. When it hit 50 seconds, I rested til the next minute started. I had to constantly watch the clock, and I had to move it to the floor for the pushups and shoulder taps. It worked, but for a few of the exercises like jump lunges, I had to guess when 30 seconds was up. I’d count how many lunges I did the first time and tried to remember how many were 20 seconds worth.