You need a axel that can move side to side (somehow) and a king arthur arm. Add an upper arm stop, just a bar across the frame in the right spot (or one of the other alternatives), and your done. I have pages detailing how to make slings slings, triggers etc. If there is some part is particular you want details about, just let me know. There should be most of the info you need scattered around the site, but if it is missing, or is simply hard to find, its a problem with the site and I would appreciate you letting me know what you are missing so I can fix the site (and thus also provide you with the info you want).
I don't generally like the idea of giving out specific plans though, as its both uninformative to people (nothing I make is hard to design if you understand the concepts behind it, and seeing plans helps little with that), but I would like to provide enough technical information and design suggestions to get someone started with their own designs.
Honestly though, I do like the FAKA (its very cool), I don't really think its worth the trouble in most cases (especially if you are rather new to trebuchets). A regular KA gets almost as good performance (range for a given CW drop and projectile), and is much simpler to build and use, has less parts, and a smaller frame. Basically, for the same amount of building cost and effort, (and less design work) you can make a taller KA and throw just as far as a FAKA. Mini KA is a nice example of an easy to make, high performance KA.