If you're not sure what you're doing with the visa application, go to these guys. I booked a half hour appointment and was given great advice on various options by Grace. I was applying for a retirement visa and was promptly collected from my accommodation at 7am two days after my initial appointment. A plush people carrier conveyed me to a bank in the centre of Bangkok where I was assisted by Mee. All the admin is completed promptly and efficiently before being conveyed to the immigration office for completion of the visa process. I returned to my accommodation just after noon that day in what was an extremely stress free process. I received my non resident and retirement visa stamped in my passport together with my Thai bank pass book the following week. Yes, you can do it yourself but will probably find many obstacles. Thai visa centre do all the leg work and ensure everything goes smoothly 👍