Obs thought that same thing a couple weeks ago. I walked through how I do it, and he's now doing it no prob as well
the key is to make yourself as narrow as possible. take your right hand and and reach across your body and grab your left shoulder, then pull very tight so that your right shoulder gets stretched out and rotate your body to the left. you should now be fairly narrow and able to see the meaty section of your lat in a mirror. now simply take your left hand and pin your right lat.
other then the glutes, lats are the easiest place to take a 3cc inject