We have to:

Applying all patches will likely take a few weeks. Make sure to join our Discord for discussion and more fine-grained updates!

See our forum post for more information: https://forums.papermc.io/threads/the-future-of-paper-hard-fork.1451/.
You can also see our previous prep work on here: https://github.com/orgs/PaperMC/projects/6/views/1
Aaand you can see the current update branch here: https://github.com/PaperMC/Paper/tree/dev/hard-fork

For our dev team:

• clone into dev/hard-fork, set cleanPaperRepo in ~/.gradle/gradle.properties to a pre hard fork 1.21.4 Paper project sources root (Paper-Server/src/main/java)
• gradlew applyPatches
• Repeat:
  • gradlew gibWork
  • Tick the entry in the per-file patches issue immediately
  • if it applied correctly, continue, else look at the rejects file and apply the missing hunks
  • open the patched files and see if they compile, if they don't gradlew checkWork to move the file to the old repo; fix up compile issues; press enter (which will copy the files back)
  • git commit --amend in the paper-server/src/vanilla/java dir
• gradlew rebuildPatches

make sure IJ detected the new git repos: