Youtube is full of videos showing how to inject oneself.

Writing a text-detection script can be done perhaps with some help from Google/Stackoverflow, but requires still some intellectual effort depending on requirements.

If you solved that problem you would be rich.

Flu shots are generally intramuscular, you literally just jab it into your bicep. A layperson can do it with high probabilities of success just by reading the instructions on the box.

Indeed, pharmacists administer these all over the US.

Tricep is the way to go here.