You can warm up by doing light weight on the same exercise you're about to go heavy on. I do 1-3 warm up sets depending on the exercise, progressively getting heavier until I'm at the weight I want to do for reps so for example today I did DB shoulder press. I normally do 30s -> 50s -> 70s/80s, but today the 30s were being used so I did 40s -> 60s -> 80s