You must apply between January 1 and March 31 of each year.
You must go to the bank or banks (if you have deposits in several banks you will request a certificate for each bank) where you have the deposits and ask for the document certifying the interest received. You will receive 2 copies of the document for each bank, one for the tax office and one for you. Don't get confused, you have to bring the one intended for them to the tax office (bring them both and let them choose)
Then you need to go to your local Revenue Department Office with those documents and photocopies of your passport (all relevant pages, visas etc), photocopies of deposit books, tax ID (if you have one). Just in case bring with you also the original bank books to show or for additional copies.
You can apply online, but there should be an employee at your local office to help those unfamiliar with PCs.
I have a Retirement visa, never had a working visa, but managed to get a tax id 10 years ago when the rules for opening bank accounts were much simpler. I have no idea if without the tax id you can get a refund, I suppose yes since I got it for the first 2 years.
You will receive a receipt confirming the successful application and a personal check will be mailed to you (in the Chiang Mai area approximately 4 weeks later) with the refund which you can deposit into your bank account.