The most important thing to remember about castling in a 960 game is that the King and Rook end up on exactly the same squares as they would in a standard game:
As always, the King and relevant Rook must not have any pieces between them; the King cannot move out of or through check; and the two pieces cannot have moved prior to castling. Same as always!
If you're having "technical difficulties" trying to castle (especially where there is only one square between the King and Rook), click first on the King, then on the Rook.
Keywords: 960, Fischer, Fischerrandom, castle, castling, can't castle