First you do 2 warmup sets, light weight, first set 20 reps, second set increase weight a bit and do 15-20 reps, without the struggle in last reps. Then you go to 3 “real” sets, where you increase the weight, so you are able to do 12 reps in first set, struggling with last 2 reps, but not compromising the form. Increase the weight a bit and do the second set, aim for 11 reps, struggling with last 2 reps, but not compromising the form. Increase the weight a bit again and do the third set, aim for 10 reps, struggling with last 2 reps, but not compromising the form.