how to install the mounting bracket for ceiling fan
The mounting screws and instructions you're looking for are staring you in the face

The fan bracket screws that go with the Arlington FB442 one-sided saddle box are pre-provisioned in the box, next to where the cable in your photo exits.  (You've started to back one out already, even.)  Once you have them out, you should be able to use them to screw the metal bracket that came with your fan into the two "pockets" just outside the sidewalls of the box that are pointed out by the arrows on the box that say "REMOVE SCREW, USE HERE", thus attaching the bracket to the same ceiling joist the fan box is mounted to.