First welcome aboard. This the best place to learn. Sadly vets aren't quite up to date on safe practices for treating feline diabetes.
There are 3 times a day that testing should be done. ALWAYS ALWAYS before giving the shot. Insulin is a powerful hormone and can be fatal. You want to make sure it is safe to give. Then always before you go to bed. For some unknown reason kitties tend to drop low overnight. So you want to make sure kitty will be safe when your sleeping.
Others will chime in to explain all the technical stuff. Right now you don't have any data. And there is some info you will need to know.
Lantus is a depot insulin so it will take a few days to fill that depot. It is based on how low the glucose gets during it's 12 hrs cycle. So for awhile you might need to get a test in between 5-7 hrs after giving insulin. Other tests can be done depending on if you work or out of house for period of time.
Insulin should be kept in fridge, it will last much longer for you. No need to throw out in 30 days if that's what you were told.
Wet pate food is best, dry food is to high in carbs and keep glucoses high.
There is much to learn and can be overwhelming at first, so ask any questions you have.