Not sure how it works, but I think there's a fair chance you'll be fined when you process your application. I forget the amount (hopefully someone here can enlighten). But make sure you have a few extra thousand with you.
You can go to immigration at Chaengwatta (CW). Or you can make an appointment at Bumrungrad hospital to go the following Saturday morning. The latter option cost a bit more. Cash only at CW. Not sure about the hospital.