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
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.