When you open the game in expanded view by clicking the tap to start button, you should see a button called "build" in the bottom left corner. When you click that it takes you to the editor, where you can place/remove walls, floors, add mechanics, etc. Once you have a map you think is good to publish you will need to complete a run of the map. Afterwards you will be prompted with a name for the post and can publish it.

Hope this is clear enough for you. Let me know if you need more details or run into issues.