Download the A Hat in Time - Modding Tools. They can be found in the "Tools" section of Steam. (Note: The modding tools are not needed to play mods, only to create mods).
Launch the modding tools.
Click "New Mod" to create your mod package.
Launch the editor by clicking "Launch Editor". Inside the editor, you can create new maps, assets, and similar.
If you're looking to create a Purple Time Rift, click File -> New Map, and select the Purple Time Rift template. If the template doesn't show up, try restarting the editor.
To Publish Your Map or Mod:
Once your map or mod is ready, click "Cook Mod".
Once the cooking process is complete, click "Test Mod" to verify that your mod plays as expected.
Set your mod's info, such as name, folder name, description and tags.
Click Submit to Steam Workshop, and wait for the transfer to complete.
Once complete, you can click "View in Steam Workshop" to view your mod.