I am a very experienced airline pilot trainer, who has been training cadet pilots for 10 years. The key to success, I believe, is to establish exactly what it is you want to achieve. By that I mean are you training people for a variety of jobs, from light single-crew aircraft to heavy multi-crew, or are you training people for multi-pilot heavy aircraft only.
If the answer is the former, then the JAA ATP(A) course is probably for you. Details of this course are available from the Joint Aviation Authority document JAR-FCL 1 and the UK Civil Aviation Authority document CAP 682.
If the answer is the latter, then you need to undertake the licence training first, perhaps a FAA Commercial/Multi/Instrument, then 'bolt on' a multi-pilot heavy aircraft course comprising groundschool and simulator training. You must, of course, ensure that this course will meet the ICAO requirement for ATPL knowledge for those in the right hand seat.
Secondly, cadets who have Bachelor's degrees make, in my experience, the best candidates for training. They tend to pick things up quickly and have demonstrated both the maturity and willpower to 'stay the course'. In terms of aptitude, then the UK Guild of Air Pilots and Navigators (GAPAN) run a course to test this.
I hope that this helps!