For sust I always use a 23g 1.5" pin and inject in the glutes - alternating sides each time. After loading the syringe you can heat the oil by putting the syringe near a light bulb. That makes it easier to inject, and slightly less painful. Hold your hand steady while doing the injection. Any movement can increase inflammation. Following the injection, massage the site for about 1 minute.
As for your knot, take a hot bath and soak it and massage the site. It will be there for anywhere from a few days to a week. The more you massage it and soak it, the quicker it will go away.
You will still feel pain from glute injections, but it's minimal compared to delts and quads.