Welcome to FDMB!
We encourage blood glucose testing as the most accurate way of monitoring your cat's diabetes. You'll need a glucometer, testing strips, and lancets. Many of us use one of the Walmart ReliOn meters, which are actually branded versions of the Arkray glucometers available from our shopping partner ADW (link at top of page). When starting out, you'll want lancets in the 22-26 gauge range; as you become adept, you may use a finer 30-31 gauge.
Additionally, urine ketone testing helps monitor for one of the serious complications of diabetes, diabetic ketoacidosis. This happens when there are high glucose levels, insufficient insulin, reduced food consumption/lack of appetite, and often some sort of infection. My signature link describes how to do that.
For diet, we encourage low carb, wet or raw foods available over the counter and have a number of lists with information about the percent of calories from protein, fat, and carbohydrate. The goal is to have less than 10% of the calories from carbohydrates - about what a cat might consume eating a mouse.