Practice, plus it’s always the same person so they get familiar with their body, unlike a medical professional who would usually poke someone they have never seen before.

Practice and despair will be my guess

A tourniquet to cause vein protrusion or employ a “doctor” - a fellow user that will administrate the drug.

As an ex junkie, sometimes we don’t always hit it. And some junkies are actually quite bad at it and need lots of help.

But, as other commenters have pointed out, it’s a combination of practice and knowing your own body well.

I’ve seen kids learn to draw and administer medications in one lesson. Not iv meds, but subQ and im, yes.

Finding a vein only takes a little instruction and then some practice above and beyond what any injection takes. It’s just a more precise version.

Mind you, finding veins on someone the first few times is not always easy. But, once you’ve found veins on a given person a handful of times, it gets easier. When it’s your own, familiarity builds up quick enough that you’ll master it as quick or quicker than tying off preparing the dose.

It is true, as others have said, that not every junkie actually develops the skills. Mind you, I kinda suck at actually finding veins, and at getting a needle/iv in. Never had much practice back when I was working because I wasn’t supposed to be doing them at all.

Every time anyone can’t find my vein i can feel it when they press on it so if i did on myself i don’t think I would miss much.

Anything you do every day multiple times a day you end up getting good at. However junkies have famously poor health and do not generally practice harm reduction. As such, needles get reused, sites are not properly cleaned, tourniquets are rarely used, veins get abused, missed, burst, or the user may simply find themselves too dehydrated, shakey, irritable, or others unable to successfully hit a vein. Some may spend hours unsuccessfully stabbing themselves over and over.

Those who do the bare minimum will generally always use fresh needles, rotate inject sites, and are usually able to use either hand to find various veins in both arms.