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:
Castling king-side (0-0) results in the King on the g-file and the Rook on the f-file;
likewise, castling queen-side (0-0-0) leaves the King on the c-file and the Rook on the d-file.
In a 960 game ‘queen side’ is always the side with the d file, and ‘king side’ the side with the e file, regardless of where the king and queen happen to start.
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.
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.