It is overwhelming at first, Rhonda, but your instincts are good. We test before giving each shot (to be sure the amount you are giving is safe) and midcycle, to see how low the insulin takes him. Knowing those two things not only show you how that dose is working, but make sure he is always in safe levels. Here is a thread with lots of info on testing:
http://www.felinediabetes.com/FDMB/threads/hometesting-links-and-tips.287/
Two units is a higher dose than we suggest at first, but testing will let you see right away how it is working.
Purina DM (wet) is low enough carb but most cats tire of the taste because it is liver based. And it's expensive with no better ingredients than over the counter food, and nothing "prescription" about it except the price. If you decide to go with cheaper food, just tell the vet he won't eat it. Purina will refund the cost.
Let us know what you need to know for testing. I'd suggest you decide where you want to test (on the bed, between your knees, on the counter). Condition him by taking him there, playing with his ears, lots of praise and then treats. Soon add warming his ears. The idea is that by the time you add the poke, he will be looking forward to the treat. The most important things for us were warming the ear enough (we used a rice sock) and using big enough lancets (25-27 may work better at first rather than the skinny 30/31 gauge humans use. And find a treat he loves, and use it only for testing. PureBites and Bonito flakes are popular at my house.