Pass boards through the hatch into the attic. Finding someone to help you will make it easier. You might need to cut the boards to get them through the hatch, depending on how long they are and how the roof of your house is arranged.
Place the boards on the joists at a 90-degree angle to the joists. Arrange the board ends so that the end of each board is directly over a joist. If the end of a board overhangs a joist, you could step on the overhang and cause the board to shift. If the insulation in your attic is piled higher than the tops of the joists, move some of it aside into the corners of the attic. Be aware that whether you move or compress the insulation, you will be negatively affecting its insulating value in your attic.
Secure to the boards by attaching them to the joists with 2-inch-long nails. Two nails at the ends of each board are sufficient to keep it from moving.