Injection without knowledge is never a good thing.
If the method is intra-muscular expect a seriously peeved snake. If you have had an IM injection you will know why, they sting and fast injection stings more.
Also, all air bubbles must be ejected from the needle and, a small "draw-back" must be performed to ensure you have not injected into an artery or vein.
Better yet, get someone familiar with injections to help you.