I used this several years ago when one of my snakes turned up with pinworm. (I dosed the entire collection, just in case). I open the mouth with a rubber frosting spatula, and insert the syringe (no needle) slightly into the throat. Depress slowly, remove syringe, and the snake will give you a horrified look as it slowly closes its mouth. (I'm guessing it tastes yucky).
I 100% tell you now, you will need a second pair of competent hands to do this, lol.