After cleaning the injection site and preparing your rig you'll want to attempt to follow the procedures I'll detail below:
Use gravity to bring blood to the limb you're going to use to inject before applying your tourniquet (tie): swing or hang your arms, make a fist, etc. Tie your tourniquet in such a way that it can be easily removed if necessary. Try to secure 'rolling' veins like those in your forearms before you inject into them. Finally, be sure not to leave the tourniquet on for too long. If you feel your limb becoming numb or notice it turning blue, undo your tourniquet and don't retie it until you're ready to inject.
INSERTING YOUR NEEDLE
Insert the needle into your vein with the needle bevel opening facing up, at a 15 to 35 degree angle, and always in the direction of the heart. The more perpendicular the needle is to the injection site, the greater chance you have of sticking the needle through the vein instead of into it.
Correct needle insertion- blood flows freely into needle:
Incorrect needle insertion- bevel on vein lower wall does not allow blood to flow:
Once you think you're in a vein, pull the plunger back to see if blood comes into the syringe. If so, and the blood is dark red and slow moving, you know that you've hit a vein. You can now untie your tourniquet and proceed to inject your drugs. If no blood or only a very tiny amount of blood comes into the syringe when you pull back, you're not in a vein and will have to untie your tourniquet, pull your needle out, and try again. If you proceed to inject without being properly positioned in a vein, you'll be putting your drugs into the tissue surrounding the vein, under the skin, or some other place. It will probably be painful and become swollen, and the effects of your drugs will come on much more slowly. You also risk abscess formation and other possible problems.
After you've successfully injected your drugs, carefully pull the needle out of the injection site at the same angle at which it went in. (To minimize bruising, you should have untied your tourniquet before you injected your shot.) Apply pressure to the injection site to stop any bleeding. Don't use alcohol pads on a fresh injection wound: alcohol will cause it to bleed more, not less.