Hello and welcome!
First off, congrats to you for getting home testing up and running on your own! It's the most powerful tool in your toolkit for dealing with feline diabetes and keeping Mr. Moonie healthy and safe, so well done getting started on that!
A couple comments on the advice you've gotten from your vet so far:
1) Lantus is an excellent insulin for cats, so well done there.
2) 2U is a bit higher than we would normally recommend for a starting dose, so I'm really glad you are home testing. We would normally start at either 1U or 1.5U (depending on the dosing method chosen, see the stickie on the two dosing methods in the Lantus stickies).
3) I'm not a fan of the twice-daily feedings for a diabetic (frequent small meals seems to work better for most cats). Especially for an underweight diabetic cat, it's OK to feed them a bit more if they are hungry.
4) the dry Purina DM isn't the greatest choice for a diabetic cat, oddly enough (the DM wet food is fine), as it is high in carbs.
We would definitely recommend switching to a low-carb canned diet. Many of us feed Fancy Feast or Friskies pates (much more economical than the "prescription" vet diets!), but there are a lot of other options out there these days. Dr. Pierson's food chart (in the yellow stickies at the top of the forum) is a great resource. You're aiming for under 10% carbs.
VERY IMPORTANT: any food transitions should be done while testing blood glucose
regularly-- before each shot, and multiple times a day between shots. Removing carbs from the diet can have a profound effect on blood glucose, in some cases dropping it by hundreds of points. Especially with Mr. Moonie on a relatively high dose, you will need to be testing to keep him safe while switching foods.
I feel as though I am just following the Dr's instructions and don't fully understand... what I'm doing.
Crista has already pointed you to the Lantus forum, there is a
ton of information there about Lantus and how to work with it to help your kitty, and I've also laid out a bunch of different topics here. All of this is a lot to take in, for most of us we just get started one small bit at a time.
One thing you can do to help us help you is to provide a bit of about Mr. Moonie (age, weight, other health issues, insulin used, your general location, anything else that might be helpful) in your signature, and also set up a spreadsheet showing the blood glucose data you already have. We are very test- and data-oriented here, you will find! We use a standard template for spreadsheets and put a link to our own cat's in the signature so anyone giving advice can easily assess the situation.
FDMB SPREADSHEET INSTRUCTIONS
OK, that's already a whole lot, so I'll end here and just again say welcome to you and Mr. Moonie!