I found this explanation on another forum. Hopefully, it'll help.
Stand with your feet apart about two feet, balance a bend forward to look... (once you master the technique, 2' is no longer required, nor is looking). Just above your penis establish a visual line where your scrotum meets your groin. Your scrotum extends either side of your penis. Just below that visual line you established inside and at the top of the scrotum is the space where your testicles dropped through (I believe it's called the inquinal canal). Gently cup them and let them find their own way as you push gently.
Please be careful doing this, as forcing them or doing it wrong can hurt very badly, or actually cause damage (as can becoming aroused while in this state.)
Yes, there are gaffs and tape, etc. I never tried the tape (ouch!) and I was never satisfied with gaffs. Just tuck it up there really well, put on some too tight panties (or hose if you prefer, but I like the non-stretch cotton panties), and enjoy the tight pants!! I wear them far too often, and never show.
Now, until you've been on hormones for awhile, you will likely (unless you're lucky) have issues with the boys falling back out periodically. Just excuse yourself to the bathroom and whip them back into shape. After awhile, it becomes 2nd nature, and you don't think about it anymore. Having said that, I still chose a bathing suit with a little skirt attached, as tucking just wasn't working in a bikini (because of the spandex material.) Best of luck!