To do a perfect Quad injection: Measure halfway from knee to hip. On the very center. Then move to the outside of your thigh, the part of the quad that "sticks out". That is the perfect spot to inject, no nerves etc.
In the glutes, put your hand on your hip with your thumb pointing downwards towards your feet on the front, spread your fingers as wide as you can on your buttcheek, pin between the little and ringfinger. Always perfect and no pain.
Delts, go to the very middle of your delt. Draw a rectangle in your mind with the top of the pyramid in the center top of your shoulder. Pin in the very center of the "pyramd". Perfect, painfree shots every time.
Thats where us nurses inject patients.