If you're looking to install a beam across two 6x6 posts, you'll need to first notch the posts. Notching a 6x6 post is a fairly simple process that only requires a few tools.
First, use a chalk line or laser level to mark where the beam will sit on the posts. Then, use a saw to cut out notches on each side of the post at the marks. The depth of the notch should be half the width of the beam plus 1 inch (this will allow for proper support).
Once both posts are notched, test fit the beam in place. If everything looks good, then proceed to attaching it with screws or bolts. And that's it! You've now successfully installed a beam across two 6x6 posts.