Sparrow

Java signature mappings for classes, fields, and methods.

Raven

Java exceptions clause mappings for methods.

Fork and clone this repository.
Change the Minecraft version in build.gradle (line 12) to the version you wish to work on.
It is recommended you update from a version with existing exceptions and signatures to lessen the workload. To do this, put this version in place of the brackets on lines 31 and 35.
Run ./gradlew setUpSource.
Import this project into your Java IDE. It will have the deobfuscated Minecraft source in the main sourceset.
Edit the source and add signatures.

This means adding generics to classes, fields, or methods, or adding throws clauses to method declarations.
Be careful not to change any class names, field names, or method names!
You may have to solve compile errors in order to save your work. This is because the decompilation process is imperfect.

Run ./gradlew save. Your progress is now saved in the signatures/ and exceptions/ directories.
Commit and push your work.
Open a pull request with your changes.