There are three ways to play from a custom position: you can either choose from a book opening, create your own custom setup, or from a FEN!
From a book opening
Click on New Game from the homepage and select Custom
Choose your opponent - You can NOT play odds/custom position games against a random opponent and must choose who you are sending the challenge to.
Click on Standard and select Custom Position / Odds
The default choice is Auto Balance for an odds game. Click here, and change it to either Book Opening, Set up Position, or FEN
Be sure that the rated toggle is off, then you will be able to see the Setup dropdown.
If you select Book opening you'll be able to choose from any of a hundred different openings - Whatever opening you pick, the game will start with those moves already played!
Create your own setup
If you want to create your OWN setup, you can choose Setup Position from the drop-down when clicking on Standard
Simply move pieces around on the board, you can drag them off the board to remove them or drag pieces from the upper right onto the board to add pieces.
Select from the dropdown whether White or Black has the first move.
If White or Black has castled in this setup, check the appropriate square: O-O, or O-O-O
When you've got the board how you want it, click Load and this is now your setup for the game challenge!
Load from FEN
Select FEN from the dropdown, and you can enter a FEN to load a position right away! If you've created a setup before, you can save the FEN to save time when using that setup again.
Check out this article to learn more about FENs: What are PGN & FEN?