I would recommend starting level one day one or L1D1. Do as many rounds as you can in 20 minutes. Follow the same Rx for Day 2 and 3.
You'll be suprised how many pullups you do if you follow the regimen. Take it easy to start. Just attempt to complete the 20 minutes, rest as much as you need, and repeat rounds as you can. The following Day one or D1 workout you do just try to beat the rounds you did before. It's slow but progressive and works well.
Say you start L1D1 and do 5 rounds. The next L1D1 try to do 6. Doesn't matter what your max pullups, pushups, or squats are just try to beat your previous rounds or time. Every Day 3 or D3 is your test. If you complete under five minutes move up a level. Simple eh? Thats what makes this program work so well. Only look forward to accomplishing the goal for that day at that level. The rest will take care of itself. And if your tired your tired. Cest la vie. But keep pluggin! good luck.