I haven't done it before but I just looked and it seems pretty straight forward.
Click on "New Game"
The type in the name of the person you want to play or their email address.
If you want to set this up as an Open Game then select that option instead (applying whatever time bank / rating filters you want). All Set Piece games are unrated.
Then below "Set Piece Game" you need to put a tick in the Advanced box. When you do this it will open up an editable board that you can edit to your spec. However, I cannot see a way of loading the layout of the board from PGN or FEN.
Hope this helps.