Basically I load as much weight as I can that allows me to get at least 8 reps before I can’t do another (once I can’t do another I’ve reached ”failure”). Next time I do that exercise, I try to get at least one more rep than I did last time (for example, if I got 8 last time I did the exercise, I’ll try to get 9 this time). I continue this until I can get 12 reps, then increase the weight. Hope this helps
I think the correct name for it is straight arm lat pulldown, I’ve just always called it overhead lat pulldown. I use the rope attachment on a cable machine that most people use for like tricep push downs and hold onto it with both hands while I stand up. I put the cable to the highest up setting and pull it down while keeping my arms straight and facing the machine. Tik tok probably has a video showing how to do a straight arm lat pulldown that explains better than I can