This page allows you to generate a game by uploading a yaml file or a zip file containing yaml files. If you do not have a config (yaml) file yet, you may create one on the Player Settings page. If you would like more advanced options, the Weighted Settings page might be what you're looking for.
If you would like to generate a race game, click here. Race games are generated without a spoiler log, the ROMs are encrypted, and single-player games will not include a multidata file.
After generation is complete, you will have the option to download a patch file. This patch file can be opened with the client, which can be used to to create a rom file. In-browser patching is planned for the future.