Attached is a document I've written covering use of the AmiBroker custom backtester interface. It covers the interface's object model, and the high-level, mid-level, and low-level backtester interfaces, with examples of the use of each. There's also an appendix on using DebugView to debug AFL code.
As the custom backtester interface relies heavily on looping, except for the high-level interface, this document is aimed at AFL programmers who are already comfortable with standard AFL, arrays, indexing, and looping. If you don't understand looping, it's highly unlikely that you'll be able to understand use of the mid-level and low-level backtester interfaces. You may be able to use the high-level interface without a knowledge of looping though.
This document may be freely distributed. It's also been uploaded to the AmiBroker Yahoo forum files area.