We went to the vet yesterday, and I was taught how to inject a cat. You pinch the scruff, insert the needle, drop the scruff and squirt in the insulin. And hope the cat doesn't squirm around.
In fact, I've been more alarmed by this than Tess - I've given her two injections so far, and she wasn't bothered by them. In fact, she's more annoyed/ disrupted by our ear cleaning session. So Tess was fine, but I almost fainted after the first injection. I'd never have been a good vet!