With the casters in their up position so the table is stable, place the 1/2 inch plywood onto the table base. There should be a two-inch overlap on each of the four sides between the table base and the edge of the plywood top.
Once this is in the proper position, attach the plywood to the table base using the 1-1/2 inch screws. Do not use any glue to attach the plywood to the table top.
When the plywood is attached, place the masonite top onto the plywood. Check to make sure the masonite is flush with the plywood on all four sides, and attach with a few 1-1/2 inch screws to the plywood base, preferably into the table base (to avoid any exposed screws under the table).
It only takes a few screws to affix the masonite to the table top. The masonite in this installation is designed to be disposable, so that it can be easily removed and replaced when it becomes worn.
Your portable woodworking table is now complete. You may wish to paint or stain the wood (other than the masonite), to protect it from moisture, dirt and grime.