There are three ways to play from a custom position: you can either choose from a book opening, create your own custom setup or a FEN!
From a book opening
From the homepage, click on 'New Game' and select 'Custom.'
Choose your opponent -- You cannot play odds or custom position games against a random opponent; you must select a specific player to send the challenge to.
Click on 'Standard' and then select 'Custom Position / Odds.'
Click 'Auto Balance' and choose 'Book Opening' from the drop-down menu. This allows you to select from a variety of predefined openings. The game will start with the chosen opening moves already played.
Create your own setup
If you want to create your own setup, select 'Setup Position' from the drop-down menu after clicking 'Standard.'
To customize the board, simply move the pieces as you want. Drag pieces off the board to remove them, or drag pieces from the panel on the right side to place them on the board.
Use the dropdown to choose whether White or Black makes the first move.
If castling has already occurred in your setup, check the appropriate box: O-O for kingside castling or O-O-O for queenside.
Once you're satisfied with the setup, click 'Load' to apply it to your game.
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?