It's tricky, only done each glute twice now. The first time I did it on each one it was an effin' horror show lol, trying to pin with one hand and needle was going everywhere, but I survived. I do them standing and also I've learned when pinning spots with one hand you need to hold the syringe between your ring finger and thumb so you can use your forefinger and middle to operate the plunger, getting that down makes it pretty easy. Also I've only done upper outer on each one which is pretty easy to see.
I think one of the biggest things for me is staying calm and taking my time, because I have a tendency to get a little shaky.