Sand all surfaces of your lumber until they are furniture smooth. Traditional futon frames go unpainted, but if you want to paint, do so before continuing.
Lay the two beams on their 2-inch edges, laying parallel to one another and approximately 30 inches apart.
Set your first slat perpendicular to the beams. Align one edge with one set of ends of the beams. Set the ends of the slat so they extend evenly on the outer sides of the beams.
Screw the slat in place using one wood screw per beam. Set the screw on center of both the slat and the beam, driving the screw through the face of the slat and into the edge of the beam. Be certain to countersink the screw head into the soft pine wood, or you risk snagging and tearing your futon mattress.
Set the next slat in place, parallel to and one inch from the first. You can use the narrow edge of a spare slat to ensure even spacing. Screw it in place as you did the first.
Repeat step five to mount the remaining slats, working until you have covered the entire top edges of the beams.
Sand off any splinters or roughness created during construction.