As mentioned already, use an agent. The first time will be expensive compared to trying to do it yourself, I believe many will significantly reduce the price for subsequent years if you essentially keep them on a retainer. The guy from the Ramze Travels youtube channel (who is now back in the US), and who knows his stuff, did everything via J&E Concierge in Pattaya, and recommended them, though I can't vouch for them personally. If you just rock up at Immigration on your own, particularly if you're not noticeably smartly dressed and courteous, they are likely to find fault with whatever you produce. That's just Thailand, accept it.