Lift the king up, drop it on the square it would usually end up on if castling 0-0 or 0-0-0 in standard chess. (called c side and g side, because thats where the king ends up). At least thats my understanding of it, or you could use analysis board and mess around with it till you figure it out.
>the king first
How do I castle when playing Chess 960 on Chess.com?
Do I move the king or rook first?
Please advise.