This blog entry is dedicated to helping you better play a Feral Druid.
The Macro is as follows:
/cast [stance:1] Skull Bash(Bear Form); [stance:3] Skull Bash(Cat Form)
Just throw those two lines into the Create Macro window (it may bleed over to a third line - that's just how it works) and you're all set!
I hope this write-up has helped to add clarification; and, as always, good luck!