Creating your own configuration of rooms is easy. Just select the 'Create' button from the Custom Arenas menu.

To add a room click on any square of the grid & the 'Choose Room' menu will appear. By default you'll be shown all the rooms you've created but you can also filter by global, trending, recent, friends and subscribed.

Once you've chosen the room you want to add to your custom arena, just click once and it will be added. If you want to remove a room, just click the square where the room is and it will be removed.

A room needs to be placed directly above the Start box in order to upload to the Workshop.

Here's a quick run through of the Save & Load panel: