Definitely do it with two people. I find the easiest way to do it is with one person restraining the snake inside a pillowcase, leaving only the injection site open to the other person doing the injecting.
For intramuscular injection: Inject in the first 1/3 of the snake. Feel for the thick back muscles that run alongside the spine, this is where you should aim to inject. Hold the syringe like a pen and inject at a fairly shallow angle between the scales, towards the head, into that thick back muscle. You don't need to stick the needle the whole way in, about 1/2 way is usually enough. Depending on what is most comfortable for you, use your thumb or other hand to slowly depress the plunger and then remove the needle. Gently rub the injection site.
Keep in mind that your snake may struggle in this time, so your helper should keep a firm grip and you should try to be as steady as possible. It's honestly not as scary as it first seems, the first couple of time can be a bit challenging but after a while you'll be a pro.