I have been doing what they call a retirement visa for 25 years. I used to live in an outer province and had to do it all myself - if I made a mistake they were always nice about helping me fix it,
At Airport Plaza Immigration, there is or maybe was, a table outside with volunteers helping people filling out forms.
Since moving to CM - I have been very happy to pay a nominal fee to an agent to let them take care of near everything for me. I use Assist Thai Visa and they have been terrific. I need to get my bank letter by myself, then go to the agents office where I sign all of the forms as they flip the pages, and then meet the agent at Immigration [usually the next day, where I am needed for 1 minute while they take my photo and that finishes everything. The agent takes away any small worry about filling in the forms properly.