shadowfacts.net/site/tutorials/forge-modding-112/workspace-setup.md

metadata.title = "Setting up the Development Environment"
metadata.date = "2016-05-06 11:16:00 -0400"
metadata.series = "forge-modding-112"
metadata.seriesName = "Forge Mods for 1.12"

Java

This series does not cover learning Java or installing the JDK. You should have the Java 8 JDK installed already.

IntelliJ IDEA

I will be using IntelliJ IDEA throughout this series as it is my IDE of choice. You can download the free community version of IDEA here. It is possible to use Eclipse if you prefer.

Forge MDK

From the Forge files site, download the latest MDK for 1.12. (Click the button with the floppy disk icon labeled MDK on the left.) After download, unzip the MDK to a new folder wherever you like. After unzipping the MDK, we can delete a number of extraneous files that are part of the MDK. You can delete every file in the folder thats not one of these:

• src/
• build.gradle
• gradle/
• gradle.properties
• gradlew
• gradlew.bat

Forge

Now, to setup Forge and create the IDEA configurations we will need, run this command. (Replace idea with eclipse if you are using Eclipse and remove the leading ./ if you are using Windows)

./gradlew setupDecompWorkspace idea

Note: This may take a while to run, depending on the speed of your computer.

Now, if everything ran sucessfully, you should have a file that has the .ipr extension in your mod folder. Launch IDEA and after doing so, click the Import Project button and open the .ipr file in your mod folder and wait a moment for IDEA to reconfigure itself for the project.

Note: If you have not launched IDEA before, you may need to go through some first time setup options beforehand.

Now that you've got IDEA setup, check out how to setup the main mod class.