Here you can find complete guide how to start writing patches (or to creating your own Scala plugin builds) to IntelliJ IDEA Scala plugin.
1. Checkout project. You can checkout from our GitHub repository: https://github.com/JetBrains/intellij-scala
2. There are few branches for different Major IDEA releases. Currently master is for IDEA 12.
3. You can use any IDEA version with any installed Scala plugin, just open Scala plugin project in IDEA
4. Now you need to check, that your JDK contains required for development JDI library, for Windows or Linux you have to add tools.jar from jdk/lib, for Mac you don't need to add anything
5. Now you need to add IDEA installation directory in right place
5a. For master branch you need to create directory SDK\LedaSDK and unpack IDEA 12 files here
5b. For Nika branch you need to create directory SDK\NikaSDK and unpack IDEA 11 files here
6. The last step is to choose 'main-scala-compiler' library in fsc settings
7. Now you can just run/debug IDEA run configuration to get IDEA with just compiled from sources Scala plugin (you probably won't get compilation from the first time, because of Scala 2.10 fsc bug, in this case you have to Stop fsc, then try again)