What steps do I need to take after obtaining my 90-day NON-O retirement visa to secure a one-year extension in Thailand?

Actually you will need to get

-Thai Number - sim in your name

-TM30 where you are staying ( to send that off to immigration for certificate of residence )

-Thai bank account to transfer in equivalent to 75,000 baht from your home bank

- get a local driver's license ( helps not required ) -there is alot to this part-

Apply for retirement visa at immigration ( meanwhile make sure your deposits are at least 75,000 baht monthly )

Your bank account is the first thing you need to get, You’ll need to get a Thai phone number, a certificate of residence (get this from immigration, at the main offices), and your passport. Make sure you take photocopies of all your documents. Kasikorn bank seems to be a popular choice at the moment.

I'd say the next challenge is getting a Thai bank account. Everything after that is relatively easy