Basically you need to prepare for and take the EASA ATPL theoretical exams, which by the way the is toughest part of the so called "conversion". You must be enrolled with a flight/ground school, you can't just prepare and the take the exams by yourself. If you're from the UK I'd suggest to get in touch with Bristol Ground School or CATS. CATS also provide a full time course, which in my opinion is the fastest way to get the ATP theory out of the way. But be advised, don't underestimate the time and effort you have to put in to pass the ATP exams, this is not an easy affair! Once the ATP exams are done you need to prepare for and take the skills test (check-ride) CPL/IR. But before you take any action I'd recommend to get your EASA first class medical. And by the way, if you use the "search function" of the forum you'll find plenty of posts regarding the conversion FAA to EASA. Good luck!