What You Need for a 10 foot long bench:
Three 3-foot long 4 x 4 posts
Post hole digger
Six bags of concrete
One 10 foot long 2x10 or 2x12 depending on the width of........well, let's not go there.
One 12 foot long 2x8's
Three 3 foot long 4x4's
Drill and 7/16 inch bit
Fifteen 5 inch long galvanized half inch lag screws with washers
Six bags concrete mix
Selecting the Site
Digging the Holes
Dig three 2 foot deep post holes 4 feet apart. Dig them wider at the bottom than at the top so the concrete will sit more firmly in the ground.
Setting the Posts
Cut the 2x8 board into two foot lengths. Cut the boards to the shape shown in the drawing. Drill two holes in the center of each board as shown. Hole the brackets up against the posts as shown and predrill into the post. Do this for each bracket.
Screw the lag screws and washers into the mounting brackets and posts. The brackets will be on opposite sides of each post running lengthwise to the bench. Tighten the screws till the wood dimples slightly under the washers.
Lay the bench plank on top of the posts and brackets. Position the plank so it overhangs the outside posts evenly (about a foot). Drill three pilot holes through the plank into the top of the posts. Screw the lag screws and washers through the top of the plank and into the ends of the posts.
Finishing the Bench
This bench can be placed almost anywhere. You can even carry the parts, precut and drilled, back into the woods and bolt yourself a bench along your favorite hiking trail. You don't have to concrete around the posts, just tamp the dirt around it with a shovel handle. It will hold well enough for a seldom-used rest stop.
You can also replace the 2x10 or 2x12 planks with a pair of 2x6's. Just leave a half inch gap between them and screw them to the brackets with galvanized screws. Don't put them closer together than a half inch or the flexing of the boards can pinch your behind between the planks.
If you cut a 45 degree cut on each corner, it will give the bench a more finished look.
You can also string a series of planks on posts four feet apart to make pew-like seats. Two boards can share a post, just screw them into the post and brackets with galvanized screws. If you angle the ends, you can even bend the rows of seats to follow the curve of the terrain.
It's not hard at all and these benches look really nice when you're done with them.