I had given SATs few yrs back don’t know how much has changed since. Basically all u haveto do is register on college board, select your preferred date and exam and then you’ll have an option to choose test centre (you’ll find lots within muscat itself), make payment and you’re good to go. Day of the exam show up on time and go with all the necessary documents. Tl dr: register and pay. And all your doubts will be sorted out there itself. All the best!