If you still have 2weeks before your 90days report, you can do it online at Immigration website, its more convenient just took photo of your last visa stamp and TM47 and upload, fill up the form online.
But if you have lessthan 2weeks already, you need to go to Chaengwattana immigration at 90days report lane counter.
If your Visa was isseud at One stop Immigration , you can do 90days at Chamchuri square 18th floor One Stop Immigration Center.