Let me post my notes. Buying a DTAC SIM at the airport seemed to be the consensus of the information I googled. I wanted to post all my notes as I learned some things doing the top up:
I bought 4.5 gigs for 1 month for 549 (or 649?) baht at the airport. It included some call credit. The airport attendant put in the SIM and activated the service and made sure it is working, which is a good reason to buy it at the airport. I bought the DTAC SIM so the phone numbers I give below are probably specific to DTAC. The happy tourist sim includes a help card with some codes you need to check balances or activiate services (DTAC: check baht balance *101*9#, check balance of internet / everything else: *101*1*9#). To use a code, you type it in and dial after typing in the #. A response will show on your phone immediately, or you may receive a response as a text message.
Had I only bought the SIM without the internet service activated, I would need to add baht to it (a top up at 7-11) and then buy a package via call code at https://www.dtac.co.th/en/prepaid/products/happy-unlimited-internet-package.html. I do not use voice calls much, so rather than buy a package I let it charges bahts when I call. But you can find packages related to calling.
Later I did not have wifi in my hotel and my data needs went up. I used up the monthly 4.5gb and needed to top-up. I went to seven eleven and ask for a top-up, 200 baht. I typed my telephone number on a keypad they pointed at (starting with 0). So now my phone has 200 baht. You can call *101*9# to verify the top-up has affected your balance.
Next I used the charge to buy more internet data. But I couldn't buy another plan package (https://www.dtac.co.th/en/prepaid/products/happy-unlimited-internet-package.html) like I would in other countries (because the plan is still active), I had to buy "topping", with toppings listed here: https://www.dtac.co.th/en/prepaid/products/internet-speed-topping.html
Toppings extend the fast data limit for your monthly plan. But unfortunately, like your monthly plan, they are still active and cannot be replaced after you consume the fast data. So you can only buy one topping, which for me was quickly consumed when my phone decided to update everything.
So I needed another topping. After adding more baht at 7-11 is when I realized you could only buy one topping. So I called DTAC support at 1678. I told her I needed more data, she was able to close my previous topping and buy me a new on (2 gigs 7 days for 53 baht, and I could check the remaining data balance at *101*1*9#
Another note: At one store, when I bought top-up, they gave me a receipt rather than charing the phone directly. To charge up the phone, visit https://my.dtac.co.th/esv/en/quickRefill. The serial number is on the receipt. The ID number asked for is part of the PIN code on the receipt. For the ID number, take the last 6 digits of the PIN code.