Pass wood boards up through the attic hatch. Having someone to help you do this makes it much easier. Depending on the length of the boards and the angles of your attic, you may have to cut them to shorter lengths to get them through the hatch.
Lay the boards down on top of the joists so the boards are perpendicular to the joists. Position the ends of the boards so they are supported by the joists. If you leave the end of a board overhanging a joist, it's possible to step on it and cause the other end of the board to fly up in the air.
Add extra security to the boards by nailing them down to the joists with 2-inch-long nails. A few nails in each end of the board are sufficient to stabilize it.