Well, since I can't quite point it out to you, you're going to have to play detective with your body in the privacy of your own bathroom. First thing: start to pee, then stop. Find where the pee is coming from by sticking your finger down there and feeling around. Yes, its' gross, yes, you will pee on your hand. Don't worry. That's what hot water and soap are for!
Start to pee again and find the hole. Should be on a little bump, usually just below your clit, under the labia. Alternatively, if you don't want to pee on yourself, feel around down there with your fingertip and do some kegels. You should feel the urethral bump move a bit.
Once you identify that spot, you can place the stp spoon part over that bump, hold firmly in place, and pee.
It will take practice, and even if you think you have it after the first couple of tries, keep doing it at home first. Just when I thought I had it down I peed myself at work. Not cool! To get it consistently in the right spot takes practice.
Also, learn to control your initial flow of pee. That way, if for some reason you mal-aligned the stp, you'll feel the pee leaking before you have a chance to embarrass yourself. Stop peeing, reposition, and try again.
I know lots of us have dysphoria about these body parts, but it really is in your own best interest to know yourself intimately 'down there' for medical reasons. It's really not all that mysterious.
Once you start T and your own dicklet grows, you'll have to practice re-aligning the stp, as the urethra will seem farther back.
Hope that helps. And yes, a good diagram will help you at least aim your fingers in the right direction. Trick is to get the flappy labia out of your way.
Jay