Use gradle tasks.
E.g. ./gradlew clean check buildPlugin, where clean and check are built-in gradle tasks,
and buildPlugin is provided by gradle plugin for building IJ plugins.
Use gradle runIde task provided by gradle IJ plugin.
It will download IDE jars for the specified version and will start new instance of IDE with the plugin.
To specify IDE version use LIVEPLUGIN_IDEA_VERSION env variable or modify build.gradle file.
Note that gradle tasks can be configured as "Run configuration" so you can run use them directly from IDE.
Just like with other IntelliJ plugins the best place to start is probably plugin.xml.
Packages are structured in attempt to reflect toolwindow UI of the plugin.
Please be aware that this is an experimental proof-of-concept code.