For a game whose loader has a mod template (Lethal Company), Develop in the sidebar starts a mod: New mod… asks for its name, a one-line description, the author, Thunderstore team and id prefix, and where the project goes. Signed in to the gh tool, Mortar can create a GitHub repository for it; otherwise it copies the template into the folder. The project builds against your copy of the game, and every build installs into a development profile (one Mortar creates, or one you pick).

Each project has Build (compiler errors open in VS Code, VSCodium or Rider at the line), Build on save, Release… (the next version, a changelog entry and a git tag; with a GitHub remote both are pushed and the project's CI publishes to Thunderstore, otherwise the release stays local and Mortar offers to pack and upload it), Game code… in the page header, and the page's Test (copies of the development profile on one computer, one hosting and the rest joining: one copy when every mod here is client-only, otherwise two, or the number last picked from its menu). A project you clone from GitHub or add from a folder is not built when it joins; its first build asks once, because building runs that repository's code on your computer. Mortar needs the .NET SDK to build and git to keep the history; the dialog says what is missing.

A mod of files only (suits, sounds, posters, paintings, TV videos, boombox songs) needs no compiler. New mod… first asks which kind to make, and offers a kind only once the loader that reads it is on Thunderstore or among your projects. The project is a folder of files with a manifest. It installs into the development profile as it is, and Edit details… changes its description, author, team and dependencies. A folder of this kind that you add or clone installs once you agree to Install, because a plugin among its files runs when the game starts. Release… tags the version (and pushes it when the folder has a GitHub remote), then Upload sends it to Thunderstore; such a project has no CI of its own.