There are guidelines published by different sources for injection doses and frequency (usually 1-2 weeks) in transwomen. If you can't find them, I can share some with you (I can PM you). It also depends on the type of estrogen, estradiol cypionate vs. estradiol valerate but studies would suggest both are equally potent, just that one of them (cypionate) lasts longer and levels are steadier.
As far as injecting goes, I personally inject in the upper outer quadrant of my buttocks, alternating left and right, every 3-5 days. Some inject in the thighs. It is usually done intramuscularly (in muscle) but it can also be done subcutaneously (in fat).
Intramuscularly, the needle is longer, about 1-1.5 inches (1 inch better for thigh, 1.5 for butt). As the solution is oily and thick, the needle gauge (size, diameter) is larger, around 21-23 gauge. Some are fine with thinner needles like 25 gauge, others draw the solution from the vial with an 18 gauge as it's easier this way and quicker.
I was deathly afraid of needles but after a year of having nurses inject for me, they taught me how to do it on my own and it's been more than a year I have done it myself. It's usually painless, very little to no blood comes out and it's really quite easy, in the end. I personally preferred it over pills but had better breast growth on pills.
Estradiol valerate comes in 10 mg/ml, 20 mg/ml or 40 mg/ml. 1 cc = 1 ml. Estradiol cypionate comes in 1 mg/ml and 5 mg/ml. The buttocks can store as much as 3 ml, the thighs, I believe is 1-2 ml, at most and shoulder, 1 ml.