Custom Missions have some things in common with mods. They both are shared as files that go in Orchid Rain - Mission X_Data > StreamingAssets. While mods reside inside the "models" folder in this directory, maps go on their own folder, named "maps". Custom Missions are comprised of two complimentary files. The first file could be thought of as a "wrapper", which contains only the metadata of your map. This wrapper file should have the name of your map, which is up to you. The second file is the map itself, or in technical Unity terms, the scene. This map, or scene, file needs to be in a folder named exactly as your wrapper file, with a "_data" added. Inside this folder will be your map file, which should be named exactly as your wrapper file with a "_map" added at the end.
For example, if your map is called "My test mission", your files should be named as follows:
You can check the existing maps available for a better example.
Once you have a map correctly placed within the game folder, you'll see it listed in the main menu when you click on the Mission Browser button.