Covering part or all of the ceiling joists in the attic is a cheap way to make use of wasted space. Installing the subfloor is the easy part. The difficulty comes from the precise measurement and cutting required and the labor of getting the sheets of underlayment in place.
Since ceiling joists are 24 inches on center and wood subfloor material comes in 4-foot by 8-foot sheets, you can potentially use a lot of whole sheets with no need for cutting. The question is, can you get a whole sheet through your attic access door?
Measure the area closest to the attic door to determine if a whole sheet can be used, or what the first cut must be. When laying out for the first sheet, keep in mind the distance between the joists as well as their thickness. Two sheets will meet over a single joist, so each will have only half the width of the joist for nailing. That means 3/4 inch per sheet.
Carry the first whole sheet of flooring into the attic. If whole sheets are too large, start with the longest piece you can fit through the door. Align the edge carefully with the joists so that you are only covering half of each joist at the edge of the sheet.
Nail one corner, leaving the head of the nail slightly above the surface in case you need to move it. Nail the opposite corner lengthwise down the sheet, being careful to check the alignment on the edge of the joist below. Ensure that the sheet is still in the correct position, then nail down all four corners.
Nail down the rest of the sheet, spacing nails about every 12 to 18 inches apart down the joist. Measure the next cut or plan the next whole sheet. Stagger the sheets where possible. When making cuts, remember you are only covering half of each joist. Repeat Steps 3 and 4 until the desired attic space is covered.