Molly was very feral in behavior....short tolerance for pats nor would she sit in your lap for cuddles...and at the vet...PSYCHOTIC by times.
When I found out she was diabetic, I had no idea how on earth i was going to manage to give her the insulin let alone do any Blood Glucose tests for safety.
maybe a few of the methods I used will be useful for you.
Started this way.....Dietary switch to canned only for 2 weeks(fancy feast classics) before starting the insulin........during this time, every evening I practiced holding her ear gently between my thumb and forefinger for as long as she would allow. A nice treat when I was through..each time. (I used these freeze dried meat treats from Purebites...can get at pet store ...no sugar added.....watch out for sliced meats and rotisserie chickens etc...they all have dextrose added which is guess what? sugar)
The ear warming thing worked especially well if she was sleepy...after a couple of days she figured out that no harm was intended and tolerated it. Next, I brought the glucometer out with a few test strips and off an on, tested myself so she got used to those beeping sounds it makes...giving her a treat after each test for myself (so she got to liking the beep sound and waiting for her shrimp)....the reason I did this was because later, when you do real testing of the cat, that beep goes off right next to their ear and depending upon how easily strung out your cat is could cause an issue.
After a few days of this, with she and I relaxing a bit about it all...I tried testing her ear using the methods recommended on this site....didn't go well the first few times because she didn't like being held still and because I was a nervous wreck about it (it helps to have a cotton ball on the other side of the ear when doing the lance....made it much easier)...and bumbling around trying to find meter and not have her shake her head before I could get the blood drop etc....
It was about 2 weeks of this when I felt I might actually be able to do it.....and lo and behold...Molly actually started to like the ritual...especially the shrimp part. For the shot part, the vet showed me how and let me practice right there in the office with water....make sure they show you how to read the marks on the syringe too, some have half unit markings and some do not. We used Lantus.
Oddly over the past 2 months that we did this (Molly's in remission now), Molly became more affectionate and cuddly...I can tell she feels SO much better.
I just tell you this so you know at first it's going to be really really hard and you're going to be so scared and stressed....after just a few weeks...it gets so much easier. Let us know how it goes!