Maybe a few LONG 2x6s against the roof at a angle,  Rope across the ridge to something stable, then a Pulley system hooked to plywood to pull the plywood into place on the roof, then tie off & go up & screw it down.

Going to be a "few trips" but you could do the hard work standing on the ground.

Got a grain truck? power pole strapped to bed as a "gin pole" & raise hoist to lift plywood onto roof could work also.

 Last word is GET A SAFTY HARNESS & USE IT when on the roof by yourself. Make sure your cell phone is sucure  also .