positioning the sub-floor
Determine the position of the shed and then prepare the ground for the sub-floor.
Make sure the ground where the floor bearers will sit is level and firm.
Use a level or a water level to ensure the sub-floor is level.
Also ensure the rest of the grade under the floor is lower than the underside of the floor joists.
Lift the sub-floor in place.
If the shed is in a sheltered or low wind zone, then no more action is required - continue laying the floor.
Anchoring the sub-floor
If you are concerned about strong winds and want the shed anchored to the ground, now is the time to do it.
If need be, dig four fence post type holes, one near each end of both bearers. Place 4" x 4" (100mm x 100mm) posts into each of the holes and bolt them to the bearers.
Fill the holes up with concrete.
Of course, once you have done that, you will also have to secure the shed from the floor to the roof, otherwise the posts you just concreted in will be useless.
One way to do that is to use metal galvanized strap, hurricane ties, or other fasteners, to tie the bearer to the floor joists, and fasteners to tie the floor joists to the bottom plates, and also
fasteners (metal strap, hurricane ties etc) to fix the top plates to the roof rafters.
Fixing the floor to the sub-floor
You can now fix (nail or screw) the floor panels to the floor joists.
Nail the floor sheets to the sub-floor members with 2 1/2" (60mm) galvanized flathead nails (or 2" (50mm) wood screws) spaced every 6" (150mm)
around the perimeter and along the join, and spaced every 8" (1200mm) along the intermediate joists.