Don't go nuts trying to inject every 12 hours on the dot :smile: It's ideal but you need to have a life :smile: Most insulins can be given +/- an hour without any issue, though sometimes it does depend on the individual cat.
Always get a blood glucose test before you inject. The number you get will tell you if it's safe to give insulin or not. Newbies are advised not to give any insulin if the number is under 200 mg/dl (11 mmol).
For the long lasting insulins, like Lantus and Levemir, you can inject before, during, or after the cat eats. I always tested, give food and while my cat was stuffing his face, inject the insulin. Many people inject while the cat is eating because the cat is so occupied with eating that he/she doesn't notice the injection.
For a shorter acting insulin like Humulin N, you want to fed half the food, wait about half an hour or so, then give the rest of the food and inject. The first half of the food will sort of cushion the effect of the insulin and keep it from dropping blood glucose levels too low too fast.