There's really no easy way of doing a custom syntax file in sublime. Basically is really the same as doing it for textmate but with some changes.
The correct name for "custom syntax or file type" is "Language grammars" http://manual.macromates.com/en/language_grammars.html
in sublime you gotta build the grammar file in XML instead of how textmate does it. Is more cumbersome but oh well. After creating the grammar you need to edit your theme and start coloring your syntax based on the scopes you define.
If I'm not too lazy in the future I might write a tutorial on how to do it. Not that hard once you get the hang of it.