I've found that it is easiest to attach the shelf by attaching the 2×2 side rails prior to connecting it to the table. To do this you'll sort of take a minor step back by removing the two lower 2×2 side rails (only 8 screws). Then drill the shelf to the 2×2's as shown below. Lift the shelf into place and attach each end of the shelf to the lower end rails using 2 1/2″ Kreg Screws. Once that is secure flip the table right side up and add the cross supports.
The final building step is to lay each plank in place. These will be cut to 20” long each. Attach each piece using wood glue and brad nails. Just make sure the length the nails isn’t longer than the width of the planks and plywood combined. Once you have finished that, you can paint or stain and add your hardware!  Because I was using pre-finished white barn wood from Porter Barn Wood, I painted the table before adding the planks.  Check out these planks….