Hmmm...
Have you received any instruction on how to give an Intramuscular Injection(IM)? 22 Gauge 1-1/4" is the appropriate length for the deeper muscles (Glutes, Legs). If your Doctor prescribed it, get his/her Nurse to show you how to self-inject and where, care should be taken around the Glutes/Iliac Crest to avoid nerve damage. Otherwise...have a skilled Healthcare provider Inject you. If you have received instruction and demonstrated with a"Live" IJ (Injection) several times, have the items you need (alcohol, band aid, sharps container, gloves-optional), then its fairly simple, if not you could end up injuring yourself and getting a possible infection.
And by the way, you should only do this if it has been prescribed by a Licensed Healthcare Provider, and you have received the proper training and instruction on how to safely perform the task. Otherwise...don't do it!