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It's not too difficult to do, the extra oil in there is a nice feeling. It stings a bit afterwards, not too painful, but kind of an annoying pain because it is constant when you move your arms.
What I did was flex my bicep, and marked the approx. peak with my finger. I then looked closely for veins, and injected as close the spot I marked, without getting too close. My arms aren't too big, maybe 17 inches, and I was planning on using a 5/8" needle, but I didn't have one, so I used 1". I loved the size it added, even if it was temp.