I tried to use a coffee can lid as an STP and I was just wondering exactly where you have to put it to make it work. The lid worked a little bit but most of everything went all over the shower. This website said to put it behind the urethral opening but I have no idea where that is. Can someone help me?

I actually don't know the exact location either, given how much I've neglected that part of my anatomy.

Uh...maybe try right behind the clitoris?

I'll try that.

I noticed that it's further back than I would have expected (I swear I passed anatomy  ).  If you're okay with poking around down there or using a mirror that might help you figure out where to place the lid. 

i was game enough to learn to pee standing up without equipment. just learned to aim. n first go hit the ceiling. but its hard to aim with an stp for me simply because certain things just get in the way..grr. so ive improvised by learning to aim and direct and now after a few weeks i can write my name on a wall. im proud of myself for getting over that. but everyones different in that region stp isnt for me but that shouldnt stop anyone frim developing their own unique and comfortable style lol.

I have a similar problem.  Sometimes I have to put my hand down and feel and put my finger exactly where my urethra is to line the STP device up in the right spot.
I don't know where my urethra is without feeling for it. 
This is how I feel for is, I put one finger back there in the general area, and then I flex my muscle down there (almost like pushing to almost start to pee, but not yet).  and then I can feel where the urethra opening pushes out.

I don't know if that works for you, but that's what works for me.
sorry if that was kinda gross

I figured it out. Lol.