Lay the bottom plate where the post will go and mark the concrete through the four corner holes. Remove the plate and drill four 3 1/2-inch holes. Lay the bottom plate back on the concrete, lining up the holes.
Screw the four 3/4-inch hex bolts about halfway into the leveling plate. Set the post flush into the leveling plate and place the plate over the bottom plate. Line up the corner bolt holes on both plates with the four holes in the concrete.
Measure the plumb using the bubble level, and adjust the hex bolts accordingly to level the post. Screwing the bolt down on one side will tilt that side away from you, while loosening the bolt will tip the post toward you. Use all four hex bolts to ensure the post is level on all four sides.
Place the washers over the four concrete bolts and screw the bolts through the upper and lower plates into the concrete. This will require a strong amount of pressure so use either a good ratchet wrench or a power wrench to set the bolts.
Slide the decorative base mount over the top of the post and screw it into place, hiding the connection between the post and the concrete.