I've given many injections on my own.. I would NOT put your thumb on the plunger as you insert the needle, it is too easy to squirt antibiotics everywhere but in the snake. Use one hand to grasp the snake behind the head, and the other to insert the needle (remember in between scales) then move your thumb to the plunger and inject the medicine. Remove needle, and release snake.

Snakes don't seem to mind the needles for the most part, it's more the holding them still part that they don't like. In fact, some snakes do better if you don't even pick them up, just stick the needle in their back when they are sitting in their tub.