I guess you can start with [1] and ask people in community, there may be a couple of simple examples to show.
[1]: https://github.com/red/red/wiki/Red---Rebol-Dialects:--Selec...
[1]: https://github.com/red/red/wiki/Red---Rebol-Dialects:--Selec...
With that said, I would appreciate if you'd listed what would you like to see in "How to bake DSL with Parse" guide of some sort, that would be really helpful when the time will come for writing tutorials and documentation.