The predefined properties are a good place to start for new fixlets/tasks. If I don’t find what I need in the OOB content, I look in the Forum archives, BigFix.me, and/or the BigFix Developer’s sites for additional insight. This usually involves using the Fixlet Debugger to work out the various kinks.
Next, I would normally create analyses. However, the Remote QNA capability is very powerful, so I have begun using it (since it is already in the Debugger) prior to creating any Analyses to validate the new content across multiple machines.