1. click the avatar of the person you want to challenge (the challenge page should then come up and show that persons username.)
2. go down the left side of the page to 'set up your own board' and click that.
3. now you are ready to set up your board. You can import a FEN or manually set up the pieces.
There are many options to use when setting up your board, so take your time and look that page over.
4. When you have the board set the way you want it, go to the bottom of the page and click 'Set Start Position'.
5. That will take you back to the challenge page and you should see your board. Finish setting up the challenge parameters, including color to play, time controls, etc.
6. Go to the bottom of the page and click 'start new game'.
That should do it. If your prospective opponent already is playing at their maximum games or has a challenge limit set, the game will be rejected,
so you may want to check with them first and see if they can accept a new game.