Testosterone can be administered parenterally , but it has more irregular prolonged absorption time and greater activity in muscle in enanthate , undecanoate , or cypionate ester form. These derivatives are hydrolyzed to release free testosterone at the site of injection; absorption rate (and thus injection schedule) varies among different esters, but medical injections are normally done anywhere between semi-weekly to once every 12 weeks. A more frequent schedule may be desirable in order to maintain a more constant level of hormone in the system.  Injectable steroids are typically administered into the muscle, not into the vein, to avoid sudden changes in the amount of the drug in the bloodstream. In addition, because estered testosterone is dissolved in oil, intravenous injection has the potential to cause a dangerous embolism (clot) in the bloodstream.
If you are going to start your first cycle soon, 'how to inject' is probably that last thing that you are worried about. You would have started by conducting research on the different injectable anabolic steroids available in the market, whichyou think can help you reach your goal. But when you have the vials and the syringes in front of you, you will surely think about how you will get the steroid out from the bottle and into your body. At this point, some people will become exasperated and even give up. Here is some information on injecting anabolic steroids.