Most grocery stores and any cooking equipment store worth its salt will have kitchen syringes available. These are like big hypodermic needles with holes all along their sides, and they are used to add seasoning to the inside of the meat.
Do not use anything with particles larger than the holes in your needle, or it will cause it to clog. (Strain the marinade before pouring it into the injector if it has large pieces.)
Load the injector with marinade.
Insert the needle into the fleshy parts of the turkey. Withdraw needle slowly while pushing the plunger, to evenly distribute the marinade throughout the turkey and avoid large pools of marinade in one place.
Make sure you inject in enough places so that the baste is evenly distributed.
I like to use a mixture of chicken stock, pureed garlic, and melted butter. Don't go too crazy with the amount of liquid that you inject. A 12 pound turkey only needs about a cup of liquid. Any more and it will just leak back out.
Check out the Marinadesection for more information about marinades you might consider using for injection basting.
Consider putting a seasoning dust on your turkey after injecting marinade. Check out ourSeasoning Rubspage for ideas and instructions!