The first thing you will need to do when choosing to rough finish your attic for extra storage space is to check the integrity and number of joists in your ceiling. The best way to do this is to crawl into the attic and to measure the distance between joists. Ideally, the distance will be no more than 24 inches. The joists should feel solid and be in solid repair. The will be the real basis for the floor of your rough finished attic.
Next, determine where, along the centerline of the roof, you will be best able to cut a long enough hole in the ceiling to load the necessary plywood into the attic. This will require cutting a large piece of drywall or plaster from the ceiling which will be replaced after the attic is completed. Cut the hole out.
Depending on the height of your ceiling, you may be able to get a length of plywood through your newly constructed hole in either 4 foot or 8 foot sections. Whatever the length it will be important that it match with the joists so that the length of the plywood will lay at centerline between the starting and ending joists when laid.
Starting in the hardest-to-reach corner, begin placing ¾ inch or 1 inch drywall on the top of the joists, being sure to position the ply board so that the center of the joists is lined up with each end of the ply board. Screw (or nail, but preferably screw) the ply board securely to the joists.
When starting a new row, stagger the boards being laid, much like you would stagger bricks when laying them. Continue in this manner until you have covered the entire floor space of your new attic storage space.
From below, repair the ceiling drywall and paint. Voila! You have just completed your roughly finished attic. For a variation on the finished attic, use ply board on the ceiling of the attic as well.