This tutorial assumes you have already

IntelliJ

1) Open build.gradle as a Project
Import
Import File
2) Use default settings
Import Project
3) Wait for build sync to download everything and finish
Sync View 0
Sync View 1
Sync Download
4) Refresh gradle project
Gradle Tab
Gradle Refresh
5) Run genIntellijRuns
genIntellijRuns
6) Fix run configs
Configs
Select Run Config
Go Edit Configs
Edit Configs
Error No Module
Module
Set Module
Save your changes 7) Run Run Debug
Run
2 Mods loaded!