Just walk in.
You need your Passport, a pen, and preferably a copy of your TM30 from your hotel.
As you enter past the initial security bit tell them you want a 30 day extension and they will give you a queue number.
Don't enter the main building (yet), go to the covered area with lots of chairs.
You need to fill out 3 forms, blank forms are available there. As mentioned above it's TM7 and two other forms about overstay. Make sure you have a pen.
There are people there that can check you've filled it out okay.
Then go down the left hand side of the main building where there is a room for photocopying. They will do all the photocopies necessary and take your photo if you don't have one.
You will need your TM30. If you can, get a copy of that from where you are staying (makes things quicker), but if not the room next to the photocopiers will give it to you, just make sure you have your Guest house/Hotel details.
Go back to the covered area, use the glue provided to stick your photo on the form, sign each photocopy, and wait for your number.
When your number comes up go to the window and they will check your forms. They then give you a new queue number.
Go inside the main building and wait for your number to come up, I think its window 10 that does extensions.
Give them the forms and 1900 baht (cash only).
After a while they will call your name and take a photo.
Then once processed they call your name and they give your passport back.
I normally get there about 10 am and it's all done by 11:30 am'ish, but it really depends on how crowded it is.
They close for lunch for an hour between 12 - 1 pm.