They have phlebotomists do it for them, then run. Source: my teacher.
Just felt like a tongue-in-cheek answer, sorry. How most of them do it? No clue. Maybe a "tourniquet" as hinted at above. I'm not that educated about drugs, but it's not that hard to go off of touch, honestly. I'm sure if they're addicts, they've done it enough for it to become second nature.