The requirements to get a multi-engine FAA ATP changed last year, with a grandfather period *if* you passed the ATP theory exam prior to 31 July 2014.
If so:
*You'll need a SEVIS approved school and TSA approval (background check, fingerprints etc).
*Flight training as required.
*Pass a checkride prior to 31 July 2016.
If not:
*SEVIS school & TSA approval as before.
*ATP Certification Training Program (ATP-CTP). ATP-CTP requires at least 30 classroom hours, and 10 hours simulator training. Six hours of the 10 must be in a Level C or higher simulator.
*ATP exam. Easy. Takes a few days of self study but it's worth while doing really well on it (see my notes comment below).
*Flight training as required.
*Checkride.
A couple of notes about the checkride:
Tolerance = 1/4 scale deflection, altitudes +/- 50'
Every FAA checkride has a ground oral portion that will include rules, procedures + questions from every knowledge area that you got wrong in the theory exam.