What is the process for applying for an over 50 visa in Thailand as a UK expat?

You head on out to your immigration office and get the hand out that lists the requirements to apply for an in country Non-O visa based on retirement for 2000baht

Most offices require you have a minimum of 15 days left on your current stamp to apply (because the application goes under review for 14 days before you go back and get the Non-O visa and a new 90 day stamp inked in.

You have to show that the funds were transferred in from abroad, you need a bank letter from the bank, update the bank book the day you apply

There is NO seasoning of the funds to apply for the intitial 90 day Non-O visa.

The 2 month seasoning comes in to play when you apply for the year extension AFTER you get the initial 90 day non-O and you have 30 days or less left on that stamp.

Go to your office and get the hand out listing the documentation requirements to apply for the 90 day Non-O

AND

while you're there get the hand out for the documents needed for the year extension (seeing as you'll be doing that after you get the 90 day Non-O and they are not the same set needed)