The project uses Gradle for dependency management and build which means it will self-provision if you don't have it therefore below directions shoudl work without any pre-requisites.
From the root of the project execute the following command:
./gradlew clean eclipse - for Eclipse
./gradlew clean idea - For IntelliJ
This will generate all necessary Eclipse/Idea files. Now you can import the project into your workspace as regular project (not Maven, not Gradle). For example, for Eclipse follow these directions:
File -> Import -> General -> Existing project into workspace
Once project is imported you need to set its Working Directory to point to the root of NiFi installation. For example in Eclipse click on the little black arrow to the right of Run and then:
Run Configurations -> (right click) Java Aplication -> New
Project Name: - nifi-ide-integration
Main Class: - org.apache.nifi.NiFi
Switch to Arguments tab and enter the location of your NiFi installation as 'Other' in Working Directory section.
One last thing is to add sources
Switch to Source tab Add -> External System Directory and browse to the root of your NiFi clone.
That is it. You can "Run" or "Debug" it.