Basically, pick the weight that you can do super slow, for about 60-90 seconds max. You should be at total failure at the end, but since the weight will be MUCH lower than 1rm, it's very safe.
I think this is a LOT more effective than doing exercises for sustained periods of time.
Someone once told me any exercise you can do for several minutes is just dancing. Not strength training. And from trying many methods over the years, I agree.
I've had friends that can squat impressive numbers try super slow body weight squats. They can do more squats with 225+ lbs at their usual pace than they can body weight super slow. Considering how much safer super slow body weight squats are (basically no risk) compared to 225+, I don't know why more people don't do it.