Hopefully you have the rest of the info, where to give it, etc.
I don't know how big your snake is. I tuck most of the snake under my arm, and give the shot as quickly as possible. Try not to stab yourself in the process. Holding the head is futile, and unnecasary in my opinion, just sort of try to control only the portion you need, while preventing the rest of the snake from flopping over to interfer.
Two people makes it easy!! I just rarely have a 2nd person available.
Otherwise(a trick I've used with other critters too) is to bag the snake, with only the lower 1/3 still poking out. Loosely rubberband the bag to keep it from coming out. LOOSELY. Then you don't have as much interferance, and the snake seems calmer over it.
Be sure you're giving the shot in the correct place, and that you are not reusing needles OR syringes. Again, don't stab yourself, don't lick the needle(yes, I've seen folks do it, no I don't know why) and don't accidently poke any parts of the snake in the process.
Hope it helps.