You need certificate of residence which you can get from immigration when you do 90 day report. Cost me 500 baht and need yellow House book or lease from landlord. Medical certificate is 100 baht from any clinic and states simply you are "strong". I just colour photocopied my license front and rear and this was fine. I went alone and managed to get by without a translator, you do the simple tests first which are unfailable. It was colour blindness, depth perception, a reaction test and the peripheral vision. Then sit and watch the 4 hours video which has English subtitles. Then get picture taken and get your license. Different transport offices may be slightly different but its all straightforward. 🙂
The international licence only lasts for 3 months then you are driving illegally. I've just got my Thai drivers license which was a full day, I watched 4 hours video 🙈 and had some basic reaction tests. Cost was 205 baht and I'm fully legal now.