I was at the Chaeng Wattana immigration today getting my 30 days tourist visa extension. I'm going to give you the exact information you need (up to date, to this day March 11, 2015), and also give you some tips, to help you with things generally speaking while you're there.
REQUIREMENTS
1. Your passport with departure card
2. The Thai visa extension request form. (download here TM.7)
3. 1 passport photo, paste to your filled out application
4. Photo copy of your passport (page with your picture) and departure card (make sure can see the stamp)
5. Photo copy of your current visa stamp
6. 1900 baht to pay for the fee
TIPS
1. Getting there. When going there, you can tell them in your best Thai accent, "pai Chang Wattana suun ratchagan" ( <<--- this one easier for me) or "pai Chang Wattana dtor mor" (sounds like dtaw maw), because my experiences in all these years in Thailand, most taxi drivers dont understand when you just tell them "Chang Wattana immigration" in English
2. Be prepared, socially, technology. It takes a LONG time of waiting, so be prepared with games, music, fully charged cell phone, and bring a charger. Outside the immigration area there are many places to sit on wooden chairs or on the floor if needed with power outlets, today i was there with a fully dead phone batt, so I was a bit upset with myself LOL
3. Early bird gets the worm! Remember 12pm noon til 1pm everyday all the immigration officers have lunch break. So better get there early in the morning and hopefully get your passport back before their lunch break.
I arrived at 11:30am with 26 queue people in front of me, and got my passport back at 3pm (so minus the 1 hour lunch break, means it took roughly 2.5 hours, so about 6 minutes per queue person if you wanna average it out)
4. Things to do if long wait. If you have a LONG queue AND get hit with the lunch break stuff like I did today, I would suggest taking a taxi to Central Plaza Chaeng Wattana, get something to eat, play video games, shop, eat, they have everything there, Pizza Company, Cocoichibanya, cheap Food Park on 6th floor, lots of Korean grills and many more