What should I do to extend my tourist visa in Phuket before it expires?

From your post, it sounds like you're wanting a 30 day extension of stay based on tourism from Phuket. Each immigration office can set their own requirements, however most immigration offices require the following:

1. Your passport and a copy of:

a. The data “picture” page.

b. The most recent visa (if applicable).

c. The most recent entry stamp or extension of stay (if applicable).

d. The departure card (“TM 6”) that should be stapled in the passport.

2. The extension of stay application form (“TM 7”).

3. A TM30 filed at the local immigration office

4. 2x passport sized (4cm by 6cm) photos.

5. 1,900 THB processing fee.