Mike, I'm sure there are some detailed posts already up here if you search it, particularly if you go back to say 2007 when they had a lot of schemes, pretty much all of which had their own pprune thread with much detailed discussion.
When I went in 2007 the first invitational stage was just with FTE, despite being located at the airline's offices. IIRC...
1 HR interview - walking through your CV, talking about you, your experiences & the examples of teamwork etc that you'd put in your application answers.
2 Maths test paper - nothing especially challenging, though you need to be able to do calculations / arithmetic, Speed/dist/time calcs etc confidently and without wasting time
3 Aptitude test - usual stuff, hand-eye co-ordination, multi-tasking, slalom type things
Good luck.