In each block, drill a 5/32-inch shank clearance hole through the tabs so that the threads of a #8x2-inch roundhead screw don't hang up on the plastic. Note that there are three styles of block: an inside corner block, an outside corner one, and a connector block. If you need to use a connector block, position it in the middle of a run or evenly space a series of connectors along a wall.
It's important to note that you don't absolutely need corner blocks when using plastic moldings. If you choose not to use them, note that the installation of plastic molding differs from that of wood molding. If you need to join two pieces of molding along a wall, use a butt joint, not a scarf joint, and apply adhesive at the seam. If necessary, touch up the joint with vinyl spackle and sand it smooth. Utilize butt joints at the ends of the molding. But eliminating the midrun block means that you need to create a temporary anchor point so you can spring the slightly overlong molding into place. This anchor point is nothing more complicated than a wood block temporarily screwed to the wall. For an extremely long molding run, leapfrog the position of the wood block along the wall to install each piece. Cut miters instead of copes for inside corners, and apply adhesive to the joint. If you sand through the factory-applied primer, touch it up with a compatible primer before painting.