Three ideas, others chime in if I'm not right on please:
1. The worst area for arteries, w.r.t. to ease of access and common places to try, is near the end of the arm is a square area b/w crook of wrist and up the arm about as wide as the wrist. We call that the "no" area at our HRC. That's the side of the wrist facing down when you write with a pen.
2. Anything that might be arterial requires, in my conservative HR perspective, an immediate trip to the ER, preferably by a sober companion who doesn't freak out and can drive safely. They can drop you off if your conscious. All US ERs must treat emergency patients.
3. Download or get a bound copy of the HR bible, "Getting Off Right." Read it often. You'll find it with a simple internet search. The harm reduction coalition has it.
Spread the word, print copies of the guide for everyone you know. I've had iv users with 30 yrs use who thought they knew everything then learned a lot when they got a copy, had it been available earlier they could have saved themselves pain, limbs, others lives, money, disease infection, shame, hopelessness and... dope. I got my copy around '03 when I joined Bluelight (my screen name charged a little in '10 so my history isn't complete (I was speedball_racer).
Be well, stay safe. And no speedballs unless you have me test it first