Do glutes first. Split your butt-cheek in to four pie pieces (line vertically and horizontally). Go for the upper/outer quadrant of that butt cheek.
Stand, but take most of your weight off the side you are injecting into. Lean against something (i.e. a doorframe) so the pressure naturally rotates your torso toward the butt-cheek you are injecting into.
When you are getting air out of the syringe, use one drop of the oil to lubricate the needle. Just squeeze the plunger and let the drop slide back down the outside of the needle. I slowly rotate the pin and even work the drop back to the tip if I don't think the pin is lubed up enough.
Hold the pin against your skin and apply gentle but steady pressure. Eventually the skin will break and the needle will glide in. Chances are, you'll have to press a little more from there, but most of the resistance is in the skin -- not deeper in.
Then, keeping the pin perfectly straight, squeeze the plunger evenly and smoothly. Once all the gear is in place, pull the pin straight out. DO NOT wiggle the needle or you'll make yourself more sore.
Also... you'll feel great after the injection, then be sore 2-3 days staring about 18-24 hours after the shot. That's just your muscle getting used to the extra volume you've injected. After 2-3 shots, that pain passes quickly.