My 9-year old cat Ernie was diagnosed with diabetes in April. It's kind of long, but here's his story so far. . .
Ernie has always been a snacker and had a weight problem, although his brother/littermate (probably from a different father) Bert has never had a weight problem. For the most part I have only ever given them Purina dry cat food and just left it out all the time for their convenience and for mine. I fed them Cat Chow for years, and then I had switched to a Purina Pro Plan type which has a higher protein percentage in an attempt to get Ernie's weight down.
When Ernie was diagnosed, his blood sugar was in the 600s. It should be more like 70-150, much like with humans. I suspected diabetes because of his decreased energy level, his increased appetite and thirst, and a greatly increased urine output which was evidenced by the amount of litter we were suddenly going through. The vet said that in cats, the urine spillover doesn't show up until it gets much higher than it does in a human, and that cats can survive (unhealthily and temporarily) with it as high as his was.
I switched both cats over to strictly wet food (Friskies Prime Filets - they'll both eat it and not vomit it). Ernie was put on Prozinc insulin twice a day. I am a nurse, and it wasn't a big hassle for me to give him the shots, although occasionally he would feel it and get fussy because he was already stressed out from vet visits and just being "sick". The only thing that made it a minor hassle was having to be here approximately every 12 hours to give it to him. Plus the insulin was somewhat expensive.
I had to take Ernie into the vet every few days to get his blood sugar checked and insulin dosages adjusted.
We went on vacation soon after his diagnosis. My mom was willing to give Ernie his shots, but because he was particularly grumpy because of all the visits to the vet and also because of the stress of our absence, I told her not to give it to him unless he was in an exceptional mood. She only was able to give him one shot, but Ernie did fine for that week since cats can
do okay on a temporary basis with their glucose levels elevated.
I did purchase a glucometer - the human ones work fine for cats, too. I just prick his ear like you would a human's finger for the testing. So I began testing his blood sugar daily or even multiple times a day and keeping a log of the time, the glucose reading, and the insulin dosage. I was very afraid of giving him too much insulin, which can be deadly and is worse than not giving enough insulin.
I am SOOO glad that I did this. By the end of May, after I had changed the cats' diet to strictly wet food and had been giving Ernie insulin twice a day, his glucose levels normalized. I am looking at the log right now, and after May 25th I did not have to give Ernie any insulin. I just would check his blood sugar a few times a month.
However, in October I noticed his energy level was going back down and he just didn't seem to be himself. So I started checking it again for a few days, and it was steadily back up into the 250-400 range. After several days of this, I started the insulin again. I had gotten kind of bad about keeping some crunchy food out again (bad kitty momma!!!

) because Bert also seemed to be losing weight which he didn't need to do. Plus, it is just a bad habit that is hard to break. Unfortunately, Ernie is a crunchies addict

.
So I put the crunchies away except for a few very small, snack servings occasionally - usually trying to do it just for Bert when Ernie was asleep. I also started the insulin again on October 19th, but I only would give it after measuring Ernie's blood sugar. After about a week of insulin and very, very limited dry food, Ernie's blood sugar normalized again. I've been checking it once a day since then and so far so good.
I've been lucky so far. But I do highly recommend learning to check the kitty's blood sugar yourself, and also getting rid of any dry food. The food issue kind of depends on the cat's other health issues, though, so you'd want to follow the advice of your vet. Luckily Ernie doesn't seem to have any health problems (e.g. kidney) besides the diabetes.