1. Download and install IntelliJ IDEA

Download and install either IntelliJ IDEA Ultimate or IntelliJ IDEA Community Edition.

2. Check out Community Edition source files

git clone git://git.jetbrains.org/idea/community.git idea

3. Configure SDK and source files

Configure a common JDK and an IntelliJ IDEA Plugin SDK. For IntelliJ IDEA Plugin SDK you should specify the directory with installed IntelliJ IDEA. By default it offers the directory of the currently running IntelliJ IDEA.

For IntelliJ IDEA Plugin SDK specify the directory with Community Edition source files.

4. Install additional plugins

Install and enable Plugin DevKit, Grammar-Kit and PsiViewer plugins.

5. Create a project

Create an IntelliJ IDEA Plugin project and specify IntelliJ IDEA Plugin SDK.