Dig two holes 32 inches apart and 32 inches deep using a posthole digger.
Put on protective goggles, a face mask and work gloves. Cut the 4-by-4-inch treated post in half with the circular saw. You should end up with two equal halves of 48 inches each. Insert the halves into the holes you have dug.
Set the level across both posts to make sure they are level with each other. You may have to add some fill back into one of the holes to make sure both posts are level with each other. Use one of the posts to tamp the dirt into place and recheck the level.
Fill both holes to the top with ready mix concrete and follow package directions for how much water to add. Check the plumb and level one more time before the concrete sets up.
Slip the post base brackets over the top of the posts when the concrete has fully set. Do not screw them on at this time. Set the 2-by-10-inch pressure treated bench seat on the posts, and using the rule, center it side to side and front to back. From underneath, mark the position of the posts onto the bottom of the bench seat with a pencil.
Remove the bench seat and post base brackets from the posts. Lay the bench seat onto the ground bottom side up so that your pencil marks are showing. Screw the post base brackets within your marks to the bench seat with 1-inch screws.
Place the bench seat back onto the posts, sliding the post base brackets over both posts. Screw the post base brackets into the posts using more 1-inch screws.
Sand the edges and top of your bench to remove any splinters and to slightly round the edges.