For a large wood structure, like a deck, to stand up, it must be supported by a series of beams. These beams are connected to posts which hold the entire structure above the ground. While you can just nail these beams in at an angle, the stability of the entire structure rests on how well these beams are secured. To provide the maximum amount of support, the beams should be set into a recess that you create in the post.
Measure the width and depth of your beam.
Make a straight line parallel to the outside edge of your 15 by 15 cm (6 by 6 inch) "6-by-6" post. The line must be in from the edge a distance equal to the depth of your beam. It must also stretch down from the top of the post a distance equal to the width of your beam. At the bottom of the line, draw a horizontal line over to the edge of the post. Repeat these steps on the other side of the post. Finally, draw a line across the top of the post connecting the beginnings of the two lines that you drew. Draw another line across the sides, connecting the ends of the two lines.
Align your hand saw with the line on the top of the post. Saw down until you reach the bottom of the lines that you drew.
Pull the saw out and align it with the line on the side of the post. Saw in until you reach the bottom of your previous cut. A piece of wood will fall away, leaving a notch.
Place your beam into the notch and clamp it into place.
Drill two holes into the beam and through the post. Make these holes an equal distance apart.
Insert a carriage bolt into the hole and hammer it through. Place a washer around the end and then tighten a nut around the bolt. Repeat this process with the second bolt.
You can install a second beam to the same post. Just cut a second notch on the opposite side of the beam, or just double the depth of your first notch.