What is the process and required documents for a visa extension at Phuket immigration?

Update after my failed attempt at an embassy letter extension at Phuket immigration last week:

Went in today and received an under consideration stamp and was told to return on September 29th

Documents needed (2 copies of each)

1. Passport info page

2. Departure card (I also had my original stamp from immigration on that copy too, not sure that it’s necessary)

3. Tm.7 form

4. Printed copy of embassy letter AND a print out of the email from the embassy the letter was attached with.

5. Was asked to fill out one more form (tm2) while they checked my other documents, which stated my reason for an extension request.

6. 2 passport photos.

Paid my 1900 baht and got my passport back a short time later.