Between the scales is the way to go. Ask your vet to give you an extra needle and syringe. Then practice on an orange or grape fruit.
The thing to remember is the quicker you do it the less pain the animal feels. Slowly sliding the needle in is going to be very painful for both you and the snake. Once you find your spot and get set GO! in goes the needle and in goes the meds. You do this a few times and you can give the shot before they even flinch. Their like "whooo what was that" and is over.