Never done insulin, but I have had to do other injections on my cats (allergies and vitamin K) along with IV fluids for a cat with kidney failure.
I was taught to make a tent by pulling up the loose skin with my thumb, index and middle finger to form a pocket between the skin and the body tissue...then inject in that pocket.
There's alot of info here...
http://sugarcat_simon.home.att.net/inject.html