I've got a few pointers for trans dudes using a STP that I've learned from trial and error and like a year of using one
Start in a shower or no pants cause it's gonna be messy no matter what when ya start.
It takes time to learn the right technique and positioning for you so don't be discouraged even if ya have pissed ya pants like 10 times this week (from experience)
Generally get close to the urinal or toilet and aim your dick down cause otherwise gravity is gonna fuck you up.
Try to control the flow so your packer doesn't overflow.
WASH IT DAILY, a quick rinse can do a lot.
If it gets like a gummy texture try corn starch, it helps.
Pissing is loud af apparently so don't piss standing up at night or at least aim to the side. (had a few awkward conversations with my dad bout that)
Put the toilet seat down, it is quick and saves you from getting yelled at.
Wear well fitting boxers or a strap cause otherwise you're gonna end up ya job with your dick sideways
Take that dick pic, cause you've literally paid for it.
Maybe carry spare boxers with you cause accidents do happen.
I haven't really used urinals but from being in a lot of men's bathrooms: don't talk to nobody, quicker in and out the better, don't peek, don't pull your pants all the way down, stay as far away from others as possible, and please for the love of God wash your hands, way too few men do.
Also please dont risk going to the men's restroom if you aren't sure you pass, the risk is small but the one time it happens might get fucked up, or end up dead. Especially if you don't live in a liberal as fuck and safe place as I do.
Feel free to add your own tips and correct me on stuff. Just trying to share some knowledge I've acquired over time. Btw Axolom is a great and relatively cheap place that I highly recommend for packers and STPs.