Players can create maps and share their work with Steam Workshop Support. Other players on Steam can enjoy these creations without having to edit any game files.
You can find player creations on the RocketLeagueMods Subreddit
The Steam Workshop Uploader Tool allows content creators to upload their levels.
The Uploader Tool will ask for a few details:
The Uploader Tool auto-detects all the assets when you hit the Submit button. Just make sure that your preview image is a sharp, accurate representation of what you've made.
You can play community-made Workshop Levels by subscribing to new arenas. This is found in the Workshop section of the Rocket League Steam Community hub.
Once subscribed, Steam will download the map automatically.
To find your downloaded Workshop Levels:
If you have any questions about Steam Workshop, check out the Steam Support Help Center.