Here Goes
This is my current understanding having embarked on the route!
1. You need to get an FAA Validation of your EASA ATPL-Free from the FAA but £44 from the CAA!
2. You need to complete the ATP CTP course. Currently CAE in Dallas a couple of others and soon ATP Flight School. This is $4995 plus flights and accommodation and is a 6 day course comprising ground school and sim training. Despite what Sporty's say you WILL NEED TSA approval which can take 30 days if you haven't had fingerprints done.
3. Having done part 2 you receive a certificate that allows you to sit the ATP written test. Most companies providing the ATP CTP throw the cost of the test in but you will need to have studied before hand.
4. Once you have the ATP pass you can then undertake the FAA flying/sim test HOWEVER in order to be eligible for this you need to accomplish 5(i think) training sessions and the FAA check-ride.
To be honest if you are doing this it is worth finding a provider that can do both EASA/FAA LPC and negotiate the extra FAA stuff as you can't do a combined check-ride anymore.
Like I say this is what I think but then it always seems to change the best advice is to contact the FAA rep at CAE/FS for the best advice