Up to 90 days before arrival, it is single entry and gives you 90 days inside Thailand. On arrival you should open a Thai bank account and deposit a minimum of 400k baht. 30 days before your stamp runs out go into your local immigration office and get the list of requirements for a 1 year extension
If you have a minimum of 500k baht in a bank account for the 30 days before the date you try to extend you have a chance. ALL other paperwork needs to be updated and there will likely be extra requirements
800k baht as a final balance on your current bank account statement. You upload the pdf file showing all transactions OR you upload an amalgamated pdf file of 3 months bank statements from the same bank account showing a minimum income for every month of no less than