A big factor is how much fat you have on the backside vs how long your needle is. You dont want to be injecting it in your fat if you have alot. If your lean just jab the f'er in, about 1-2" down from top of hip bone on the outside of your butt cheek is usually the best. That way if you have any PIP your not sitting on or very near it. If you've got some fat back there then put your weight on the foot of the side your going to inject, flex your glute and push your hip out and feel for where the fat is thinnest, usually the 1-2" from top of hip outside flank. When you inject tho, have your weight on the opp side so your glute is relaxed and not flexed, easy to just keep your knee bent alittle with just the ball of your foot resting on the ground. Use a sharpie and mark a bulls eye if it makes you feel more confident. I'd use a 1.5" needle minimum and make sure you dont jab it in to the hilt, not saying you will, but if you happen to break the needle off you'll have something to grab onto to pull it out. Depending on the viscosity/weight of the gear type, I'd use a 25g.
Hope this helps. Should have 0 problems, glutes are easy.

You can lie down but no seat down lol you will be seating on the needle. Lie down turn on the side like a whale and pin. Standing works much better though you only need one hand to pin anyways