Step 1: Open your command-line and browse to the folder where you extracted the zip file.
Step 2: Once you have a command window up in the folder that the downloaded material was placed, type:
Windows: "gradlew setupDecompWorkspace"
Linux/Mac OS: "./gradlew setupDecompWorkspace"
Step 3: After all that finished, you're left with a choice.
For eclipse, run "gradlew eclipse" (./gradlew eclipse if you are on Mac/Linux)
If you preffer to use IntelliJ, steps are a little different.
1. Open IDEA, and import project.
2. Select your build.gradle file and have it import.
3. Once it's finished you must close IntelliJ and run the following command:
"gradlew genIntellijRuns" (./gradlew genIntellijRuns if you are on Mac/Linux)
Step 4: The final step is to open Eclipse and switch your workspace to /eclipse/ (if you use IDEA, it should automatically start on your project)
If at any point you are missing libraries in your IDE, or you've run into problems you can run "gradlew --refresh-dependencies" to refresh the local cache. "gradlew clean" to reset everything {this does not effect your code} and then start the processs again.
Should it still not work,
Refer to #ForgeGradle on EsperNet for more information about the gradle environment.
Tip:
If you do not care about seeing Minecraft's source code you can replace "setupDecompWorkspace" with one of the following:
"setupDevWorkspace": Will patch, deobfusicated, and gather required assets to run minecraft, but will not generated human readable source code.
"setupCIWorkspace": Same as Dev but will not download any assets. This is useful in build servers as it is the fastest because it does the least work.
Tip:
When using Decomp workspace, the Minecraft source code is NOT added to your workspace in a editable way. Minecraft is treated like a normal Library. Sources are there for documentation and research purposes and usually can be accessed under the 'referenced libraries' section of your IDE.