What insulin did the vet prescribe? ProZinc (a pet insulin) and Lantus (a Human insulin) are popular and work well.
What is your cat's starting insulin dose?
Hometesting tips:
https://www.felinediabetes.com/FDMB/threads/hometesting-links-and-tips.287/
Few people are pros at testing from the get go. It takes a bit of time to learn and figure out what works best for you and the cat. Warm ears bleed best so if you haven't done that yet, start doing so. The "rice sock" method is popular: Take a small old cotton sock. Fill with ~1/4 cup or less of uncooked rice or dried beans. If you have a really small sock (child size or baby size), use a few spoonfuls. Tie the sock closed. Heat in the microwave until just barely comfortably warm for you to hold in your hand. Try 10 seconds to start and add more time if needed. Apply the warm sock to your cat's ear. Make sure it's not too hot otherwise you'll burn your cat's ear. If the sock is too warm, your cat will flinch so let the sock cool a bit. Wrap the sock around the edge of the ear and hold in place for a minute if possible. Then hold the sock inside the cat's ear to provide a firm surface against which to poke the lancet. You should get blood. If you get too much, not a big deal. Get what you need onto the test strip and hold a tissue to the ear to stop the bleeding. Not enough blood, try "milking" the blood drop to help it bead up more. If you're using a lancet device, adjust the depth setting so you're using the lowest setting that will give you blood.
Tip to prevent the meter from giving an error message if you don't get blood onto the test strip before it errors: Stick the test strip partially into the meter, just far enough to hold it in place but not so far that the meter will turn on. Set aside within easy reach. Warm up the cat's ear. When the ear is ready, push the test strip all the way into the meter to turn it on and show the ready for blood icon whatever it may be. It takes a second or two. Poke to get blood and by the time you have a good blood drop, touch the test strip to the blood and let the meter do it's thing while you reward the cat and stop and bleeding with a tissue. I can push the test strip into the meter with one hand. I kneel on the floor to test my cat so I brace the meter against my knee while right hand pushes test strip in while the other hand is holding the rice sock on the ear to keep it warm. If you're not that coordinated, you'll need to use both hands or and extra set of hands and somehow try to keep the cat from slinking away at the same time.
The AlphaTrack test strips are expensive. Most people use a basic Human blood glucose meter. The Relion (Walmart brand) is popular and the test strips are affordable. I'm not sure if you can buy this brand online if you don't have a local Walmart store. I personally use the AccuChek Guide. It's a brand name and widely sold online and at store pharmacies.