construction lumber has rounded edges
The rounded edges on construction grade lumber are what makes it look like a picnic table, so you've got to remove them. You can rip thin strips off on the table saw. You can rip thin strips off using a straight edge guide and a circular saw or a router.
The router will take a few passes for each edge.
You can either edge glue and clamp them or spline them with or without out glue and have a "loose" joint.
You can also drill across the width of each plank and insert threaded rod to draw them together. this would require some considerable accuracy to get the holes all aligned accurately and at 90 degrees to the edges. It would also require a long drill to make it across the width of a 5" or 7" wide plank.
The planks will expand across their width, so only fasten them securely in the center to any cross members on the base, and allow slots for expansion out toward the sides.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)