Yes underneath the glans. I'm not cut so I leave around 1 inch gap underneath the ridges of the glans otherwise the skin hurts. Become aroused first and then put the rim on. Its best to loop the rim so its tighter. You can use a bit of padding if you like, no problems. Yes then enter the pump erect or semi erect. When you now pump, what will happen is the glans is being FOCUSED on as-well ... ISOLATED if you will and being enlarged like if it were in a head pump itself. The constriction from the rim engorges the glans and hence this is focused on from the internal pressure.
You may get discomfort at first but persevere and after a few sessions you will be really pleased. Let us know how it goes.