For the ATP you'll need to have your CAA verify your license to the FAA, complete a certificate training program, pass the written, complete an approved training program if the aircraft requires a type rating and pass a checkride. You'll need a second class medical before you get in the airplane.
However since it's not an air carrier flight you don't need an ATP. You'd still have to have your CAA verify your license but you wouldn't need the CTP. You would have to pass both a commercial and instrument written and a commercial and instrument checkride.