1. Press your testicles into the canals. They may be difficult to fit, especially if you arent on hrt or havent experienced testicular atrophy from hrt. If you feel any pain stop immediately. These canals are not stretchy and can be damaged. Similarly to step 0, it may be easier to do this laying down.
2. Pull your penis back towards your anus, almost pinching it between your butt cheeks. Optionally you can wrap your scrotum around the shaft of the penis, or tuck it back with the penis.

If i need a bulletproof tuck and am using tucking panties, i pair them with heavy duty compression top pantyhose, which hold everything almost too well.

For this you will need to shave your pubic hair and potentially your shaft and butt cheeks.

As per tucking panties, do the general principles, but let the penis itself hang for now. (Also i have only been able to do this standing).

Once you have the testicles in the canals, wrap your scrotum around the shaft of your penis and then wrap it in a single strip of tape, to hold it together as a single unit. If you do this just right it will look like labia under your panties when done. It should kind of look like a hot dog in a bun.

Now do the actual tuck by getting two strips of tape long enough to go from your pubic bone to your butt cheek, and apply them to your skin on or above your pubic bone at such an angle that they cross and make an X over the shaft of the penis. How far forward/back on the shaft is up to you, youll have to play around with it to find what works best. The goal is to have just the head poking out from the center of the X, as this will allow you to pee without un- and retucking. Often the tape wont hold for multiple adhesions, so this is important.

Now pull the penis back, while making sure not to let the tape adhere to anything. Once its in position, apply the tape to your butt cheeks to hold everything in place.

There is an alternative to this if you only have one wide strip of tape: find where the tip of your penis would hit the tape when tucked, and cut a vertical slit there. Place some paper over the adhesive directly around the slit. Now tape everything up as normal (without the X of course) and make sure the tip of your penis is pulled through the slit before adhering the tape to your butt (in this case your butt crack).

This all allows you to pee while tucked, though you may need to fish around and pull your penis head out. Because of how muscles and the urethra become oriented while tucked you will need to wipe and may need panty liners in case of leakage. This is entirely ymmv territory, ive spoken to girls who need liners and girls who stay bone dry.