Like somebody mentioned a few posts earlier, the easiest way to inject in your arm by yourself is to use the frame of the door. Stand next to it and use the doorway frame to push out the back part of your arm so you can inject. I have also used the back of a chair if I'm sitting down, just use the chair back to push out the back part of your arm and inject.
I am a person with diabetes, NOT a diabetic!!!
What I do is sit down, put my knee up, and push my arm up with my knee.
And uggghh, I hate pulling my shirt up to inject in my stomach only to have my friends see an ugly bruise sitting there!
I've done it that way too, using my knee.