To make a cross court simply orient your body so that your belly button is directed near the front corner.
The ball must bounce in the back corner and stay there.
It is better to use it when in an advantaged position (when you are in front of your opponent)
To make a good cross court you can aim at the center of the front wall, this way the ball will go straight into the opposite side's corner and if your ball has good depth it will stay there.
Use the cross court a few times every rally but not too much, instead make drives all the time. This is because if your opponent get used to you doing cross court he will forestall them and make you pay.
Instead if used sparingnessly you will surprise your opponent and therefore score a point.