You apply online with VFS. The application is then handled at the office you select from this list, which you will then attend to submit hard copy paperwork and get the extension sricker in your passport. Did it myself yesterday at CW, no issues at all, they even re-did a couple of copies where mine were not to their liking (despite being "pre-approved")
Bangkok
Chiang Mai
Chonburi (Pattaya)
Koh Phangan
Koh Samui
Nonthaburi
Pathum Thani
Phuket
Samut Prakan