Continue framing a freestanding deck with the beams, headers, and outside joists.
Mark the beam where it will overhang the posts on both ends. In this case, the beam is marked 3 feet from each end. Then have someone help you place the beam on top of the posts. For the longer beams, you may need two helpers. Make sure that the beams are flush to the posts on both sides.
Check the beam for level. If it isn't, trim one of the posts. If trimming will lower the deck too much, cut and install a new post; do not shim the beam to level it. Attach the beam to the post with a post/beam connector or with T-brackets and joist-hanger screws on both sides.
Cut the two header joists to length. Use the straightest boards possible. Clamp them together on a flat surface, side by side, with crowns facing in opposite directions and the ends perfectly aligned. Mark the headers with the joist locations indicated on the framing plan, 12 inches apart in this example. Every 12 inches, make a V-shape mark. Draw square lines through the marks as shown. Draw an X on the joist side of the line.
Cut the outside joists to length -- usually the width of the framing minus 3 inches for the thickness of the two headers. On a flat surface, assemble the outside joists and the headers. Make sure the headers face each other correctly with their crowns up and the Xs on the same sides of the layout lines. Use a framing square to check that the frame is square. Drill pilot holes and drive nails or screws to attach the boards at the corners.
With a helper or two, lift the frame and set it on top of the beams. Measure the frame at all four points to make sure it overhangs the beams by equal amounts on all sides. Check again for square with a 3-4-5 triangle. Attach the frame to the beam with a seismic tie or similar hardware at each joint.