No step-by-step here, since it should be fairly obvious what I did. The parts list breaks down as follows:
•6 48" steel fence posts
•2 60" steel fence posts
•8 14mm x 1.5" wood screws
•4 12mm x 1" machine screws & nuts
Total cost was about $35.
The topmost and bottom-most holes in the 60" posts were 44" apart. Using the screws and nuts, I put the top hole of a 48" post on either end of the 60". On the 48" post, there is a bottom hole and a punched out tab to hold the fencing material. I bent the tab down with a flat head screwdriver to make a second hole. Using the hole and the tab 'punch out', I screwed the bottoms of the 48" posts to the rafters; they hang upside-down in pairs with the 60" posts as cross-pieces. My rafters are 24" on center, so I hung them on the insides of the rafters to that a 48" section of ply would have some overlap.
I cut a 44" x48" section from 4'x8' sheet of 3/4" plywood. I took the two remaining 36" posts and screwed them to the ply at about 1/3 intervals. This was to make the ply more rigid. I put the ply onto the crossmembers with the reinforcing posts up so that the ply - and the load - would distribute across the entire crossmember. Then, I loaded it up with crap.
The ply is sagging under the load, which is significant! There is easily a couple hundred pounds of miscellaneous 'garage stuff' up there. Other than that sag, I'm very happy with the result; it was quick, easy and cheap!