Meat injectors are basically a hypodermic needle with a large gauge needle. Use this syringe to place small amounts of sauces into thick parts of any meat before cooking. Whether grilling, smoking, frying or roasting a turkey this is the best way to get extra moisture and flavor into the deepest parts of the meat.
To make this work and to get the most from your injection sauce, start with a liquid that doesn't contain anything that might clog up the needle. Avoid flaky herbs, crushed garlic or anything else that might have a tough time getting through the needle. Good things to look for seasoned oils, vinegars, finely crushed spices, wines or beers.
Secondly, make sure to spread out your injection pattern. You want to get small amounts of the solution into as many places as possible. Also, aim the needle to the middle of the meat. If you over shoot, the sauce will just slip through to the other side of the meat. If you don't get it in far enough it will seep out the hole it went in through. One tip is to think about making 40 injections instead of 4.
On a final note try not to overpower the flavor of the turkey. You might be tempted to add a lot of hot sauces or cayenne and end up with a turkey too hot to eat. Go with subtle and mild flavors or use strong flavors in small amounts. Remember you want to enhance the flavor of your turkey not cover it up.