Jelq first or take steps to make sure you don’t get hard when attaching the hanger. But even if you do now, you just have to be patient and wait for it to subside before tightening and after some weeks it will become easier.
Tighten the hanger till it meets resistance, use your hand to squeeze the blood out. I then tend to push the hanger forward so it’s right behind the head, this tends to move a bit of blood to the head, so I then squeeze that out then start tightening more and make sure my shaft is suitably in place within the hanger then I just keep tightening and tightening till it’s really uncomfortable and during that kind of pull on the hanger. When you add the weight after a minute or so the discomfort goes away and you tighten a little more.
The main thing is to not allow the hanger to slip whilst hanging, that way no extra blood will be pushed into the head. Make sure your wrap is the right thickness for your hanger setup and just tighten up really well.
I have to say, I don’t think you’re supposed to hang right behind the head though. I just do it because I have a lack of shaft skin and am condition well so can take that pressure, at least at the weight I’m currently using which I’ve just put up to 8lbs. I may need to refine where the hanger is later though, perhaps I’ll have more shaft skin by then.