I think you should try and then ask more specific questions. Flex SDK is required even if you are not going to use Flex framework, because it contains ActionScript compiler, debugger and required AS libraries. Some information is here http://blogs.jetbrains.com/idea/2009/07/creating-pure-actionscript-project-in-maia/ Screenshots are a bit outdated but no conceptual changes in it in latest releases.
I think you should try and then ask more specific questions. Flex SDK is required even if you are not going to use Flex framework, because it contains ActionScript compiler, debugger and required AS libraries. Some information is here http://blogs.jetbrains.com/idea/2009/07/creating-pure-actionscript-project-in-maia/ Screenshots are a bit outdated but no conceptual changes in it in latest releases.
Perhaps I should have titled my question 'how to migrate from Flex or Flash Builder for all your actionscript-flex projects'.
Here is one great tutorial in getting IntelliJ working with the Flash IDE
http://www.rossgerbasi.com/2010/03/intellij-and-flash-cs4/
By the way IDEA can automatically inport Flex/Flash Builder projects. See File | New Project | Import Peoject from external model | ...