Using mod-uploader.jar
In the install directory of Slay the Spire, there is a file called 'mod-uploader.jar'.

Usage: java.exe -jar mod-uploader.jar [-h] [COMMAND] -h, --help Prints this help text. Commands: new Creates a new project workspace. (Start Here) Example: java.exe -jar mod-uploader.jar new -w mymod upload Uploads the contents of the mod workspace to Steam Workshop. Example: java.exe -jar mod-uploader.jar upload -w mymod

'new' will create a subdirectory called 'mymod' with some files in it and instructions on use. You will need to copy your mod to the 'content' folder within and add some store metadata to config.json.

'upload' will publish your mod to the Steam Workshop.