Each dumbbell should be somewhere near 25% of your 1RM (200lb barbell strict press, use 50# dumbbells)
Do a max effort set, put the dumbbells down and take 15 deep breaths, do another max set, put the dumbbells down again and take 15 deep breaths, do one more max effort set. Three sets total of as many reps as you can do with 15 deep breaths as rest. If you do 10 or less reps on your first it’s probably too heavy, 20+ too light. Each set will obviously get much much harder. Try not to overextend too much. Work on keeping your midline tight and ribcage down when they get really hard. Would rather see a set of five with a flat lumbar spine than ten with the banana back.