@Kim *******
Kim, I have been coming in and out of Thai for 25 years (married to a Thai national, we have daughter, own our condo in Bangkok).
Now, that I am retired, I went through the process for Extension of Stay based on retirement.
Beleive me, if you are in Thai first time, you won't be able to get your head around through all the paper work.
With eVisa 90 days, an agency would charge you 34,000 THB and you get it all sorted out within 2 days and your stay extension stamped in your passport. That includes multi entry, you can come in and out of Thai as you please
Those telling you can do it yourself, they may be right but not your first time.