There are other threads about this. You go to your local ampur and get an updated marriage certificate -Kor ror 2 I think it is called for 100b or so. I do not think you can get a visa with a foreign marriage certificate unless you have registered the marriage in Thailand. You only need a signed copy of your wifes id card. You do not need her passport or a letter but if you want to cover everything bring one. They ask for proof of 20,000 b in a foreign bank but they did not take mine. The shop across the road prints a form for you and you write multi-entry on top of it and make sure they take the 5000b. I went there last November.