You'll need a copy of the embassy letter (if still being issued), and the forms found at
***************************************************************************
**********
**********
*****
*****
*****
3/permalink/
*****
*****
*****
5
And
***************************************************************************
**********
**********
*****
*****
*****
3/permalink/
*****
*****
*****
2
You'll also need the passport photos, and 1,900 THB.