Locate the desired spot for the ceiling fan. Insert two 16-inch 2-by-4 planks in between the I-joists, tapping them into place with the rubber mallet. Mount these vertically and tight against one another, so the short sides (the 2-inch sides of the 2-by-4s) are facing down. The junction box will be mounted to these pieces.
Screw in at least four wood screws per plank from the side of the I-joist into the ends of the wood pieces. This is done with the drill and Philips-bit attachment.
Screw in the junction box with four additional wood screws, centered on the 2-by-4 planks. Route any 110-volt wiring through the side of the junction box, and cap it until you're ready to hook up your fan.