I've been packing off an on for a little over 4 years now, and here's what I've learned:
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Get yourself a nice harness. Some people go without a harness and just kinda stuff the STP in their pants and I guess it just magically stays in place all day, but I've never been that lucky. I swear by Cake Bandit's harnesses but there are a bunch of different types out there to experiment with. Hell, if you can sew elastic straps together you could even make your own. Some people also use medical-grade adhesives to keep their STP essentially glued in place (typically these STPs come with a large flap you stick to your groin). I've never tried these because I'm a hairy motherfucker and I don't want all my belly fur ripped out every time I have to remove my STP, but I know people who swear by 'em.
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Work your way up to "prime time". Start completely naked from the waist down holding your STP in the shower. You'll probably make a mess your first handful of attempts, and that's okay. It's a learning process. Once you feel very confident you can use the STP in the shower, move up to wearing your harness and/or underpants (still in the shower). Once you've learned how to do that with no messes, then work up to in front of a toilet. Then, in front of a toilet while also wearing pants. This whole process might take weeks, or even months, and that's okay. You're basically potty training yourself all over again, don't get discouraged.
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Clean your STP! Run some water and a little soap through it at least once a day. Check with your manufacturer, you might also be able to boil it or throw it in the dishwasher to get it extra clean.
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I've gone through an Axolom Squire, EZP Junior, an STP Freely, and a Spouti trying to figure out what I like the best. Of all the ones I listed my favorites are the STP Freely and the EZP Junior. The Squire is great too, nothing really against it, I just prefer the similarly-designed Freely a little more. I got the Spouti on a whim and could never really get it to stay in place reliably, which is a shame because I was really interested by how minimal it was.
Just keep in mind that everyone's anatomy is slightly different and what works perfect for Person X might not work for you and vice versa. Another thing to keep in mind is size. Are you getting an STP that can also be used for sex, or are you just getting one to pee? If it's the latter, maybe you don't need something with a super long shaft. The longer it is, the more material you're stuffing into your pants which might be uncomfortable.