Usually, you should not administer any medications without prior consulation with a veterinarian or a rehabber who is experienced with the process. Giving the wrong dosage of anything can seriously harm your turtle. The best way to learn to administer medications is by being shown by someone who knows how to do it. Ask an experienced turtle keeper or your veterinarian for instructions and demonstration.
Vitamins and supplements should be given with food, and most turtles will eat them willingly.
Injections are generally given in the front half of the body. Your vet will instruct you on where (usually a front leg), how (subcutaniously (under the skin), intramuscular (into the muscle)). Follow those instructions. It is important to immobilize the injection site. One way to do this is by wrapping the turtle into a towel (including the head if it tries to bite).
Oral administration can be a challenge. If the medication can be given as part of the food or hidden in a piece of food, that is ideal. Most turtles will stick out their head and open their mouth when a favorite morsel is waived in front of them. Drop or push the food gently into the mouth as far back as possible. If it’s placed far enough, the turtle will swallow it.
if the turtle is not eating voluntarily, you will have to restrain the animal, open it’s mouth (a credit card or small hemostats work well), and push the medication in gently.
If you cannot do this easily, have someone experienced show you how to grasp and hold the head and open the mouth. While turtles are not fragile, doing this wrong can injure your turtle!
The easiest way to administer ointments is with a cotton swab. Dispose after use and wash your hands. For water turtles, keep them out of the water for at least an hour after administering external medications. Often it is best to keep the turtles in a dry setup and soak them one or several times a day instead.