Get your TM30, if you don’t have it yet. They don’t usually ask for it, but always good to have & staple into your passport so you don’t lose it. TM47 form filled out.
You can also do it online after your first notification is done in person. I do it 1 week in advance in case there are any problems:
You need to apply for a new visa while in Singapore. They don’t do re-entries in VOAs, visa exemptions, or tourist visas. You could try for another VOA or visa exemption when you return, but depending on your history of travel to Thailand they might turn you away. Max visa exemption allowance:
Online it says you can extend the VOA for 7 days, but I’ve heard you can extend up to 30. Never done it myself so not sure. For extensions go to immigration & apply. Application form will be available at the office. Fee is 1,900 baht. Bring passport, copy of photo page & current visa stamp. Copy of your arrival card. Copy of TM30 (talk to your accommodations about it, they’re supposed to file for you & can be done online so don’t let them tell you that you have to go do it).