FUCK. so i should be injecting only in the glutes?
You can inject into the delts, quads, pecs, tris, bis, calves wherever theres a large enough muscle group to take the amount you are injecting. Generally I wouldn't inject more than 1cc into delts, calves, bi's or tris, glutes and quads are about the only ones that can take more than that optimally.
I've also read that the glutes are the only area where you'll get a 90+% absorption rate, anywhere else and you are looking at 70-80%. Rotating each glute is fine so you hit one glute on Monday and the other glute on Thursday, same rotation for the following week. It's always good to have other sites you are comfortable with as well once you get into mixing a lot of compounds, but you should be sticking with one for your first 3-4 cycles IMO.
so just out of curiosity would someone benefit more from injecting in the tightest skin over muscle? for me would be the bicep?
The "tightness" of the skin is irrelevant as long as you are getting into the muscle. This is difficult to do if you are injecting yourself, but you want to basically spread the skin apart very tight by placing your index finger and middle finger together (like you are going to finger fuck a girl) over the skin, then press against the skin and spread the fingers apart so they are in a V shape, then inject into the area between the fingers where the skin is taught. This thins out the outer layer of skin and helps get the injection into the muscle.