General plugin development information you can find here: http://confluence.jetbrains.net/display/IDEADEV/PluginDevelopment
This page contains short guide about how to develop for Scala plugin extensions. Mainly it caqn be important if you have your custom compiler plugin and you want to have to see similar error messages in IntelliJ IDEA Scala editor too.
For the first time you can just download demo project here: PluginForScala.zip
Then follow few steps to setup it:
1. Setup IDEA SDK: Project Structure -> SDKs -> Add New SDK -> IntelliJ IDEA Plugin SDK:
2. Setup scala-plugin global library (you can choose scala-plugin.jar from your plugin installation).
3. Setup plugin_scala-compiler library (you can choose your own compiler, but versions should be the same, the best way to checkout Scala plugin sources and use compiler bundled with it)
4. Be sure that your libraries are "Provided"
5. Now you are ready to run "PluginForScala" run configuration. However it will not work, because your debug IDEA will not contain Scala plugin.
5a. You can install Scala plugin using plugin manager.
5b. You can manually put Scala plugin to Debug IDEA config directory, to find it check you IDEA SDK:
5c. You can add -Dplugin.path=Path (This path must lead to Scala plugin directory, which contains lib directory, which contains all Scala plugin jars)
6. In Debug IDEA you can create new project and test new functionality, current version contains inspection, which highlights all variable definitions as error (with quickfix, which changes it to val)