Hotline Miami 2's long-awaited level editor is finally available in beta form, and can be installed right now by accessing the game's properties in Steam and selecting 'editor_beta' from the drop-down list.
The beta, currently only available on Windows - though it will come to Mac and Linux eventually - lets players create new levels and campaigns by giving them the ability to customize just about anything.
As it stands there's no way to share created levels using the Steam Workshop. Until Workshop support becomes available Hotline developer Dennaton suggests sharing your levels using "online file storage provider, e-mail, or other any other way you can think of."
The easiest way to do that is to select a finished level or campaign in the custom level selector, press 'O' on your keyboard to view the level's files in Windows Explorer, convert the folder into a zip, and then share it with the world.
There's a lot to take in, but Dennaton has explained how to opt-in, and opt-out, of the beta over on his blog. The developer has also created an instruction manual explaining the basics, which can be grabbed here.
A more in-depth manual will be made available soon, but that should be enough to get you started.