This is definitely dated material but should get you in to the neighborhood.
Use the link that @dicko provided as well as other pages on that wiki to get newer information such as options and to check if any dialplan functions have been removed or replaced.
Other reading as it relates to using custom dialplan with FreePBX can be found here
Note this is also in need of updating because things like macros have gone away.
may be more helpful too for things like dialplan writing but will be less helpful for FreePBX integration.