Set the two side pieces roughly 6.5 feet apart from each other. This is roughly the size of a king bed, which is 78 inches wide. Most king bed frames have a middle support due to their size. Most bed frames for other sizes do not need middle supports. Since a king bed is the size of two twin beds side by side, a middle support is often needed to support the weight of the two box springs and mattresses.
Pull out the extensions of each side piece; these pieces connect the sides to the middle piece. There are two extensions per side piece, and they pivot and lock to create a 90 degree angle. When pulled out, the side piece has two extensions at 90 degrees each, one at the top and one at the foot of the bed.
Set the middle piece in the middle halfway between the two side pieces. The extensions will connect to the middle piece with adjustable clamps to hold the bed frame together. The middle piece should have a set of wheels on each end so that you can move the bed easily when necessary.
Attach one of the side extension pieces to the middle section, and slide the locking clamp so that it connects both the pieces. Screw the flanged nut until the section is somewhat tight, leaving a little room for movement as you will want to adjust the frame to fit your mattress. Do this for all four extensions so that the frame of the bed is together, but loose enough for quick adjustment.
Place the box springs onto the frame. Adjust the frame to fit the box springs, and then tighten the four clamps fully until the frame is secure. You may have to reach under the box springs to do so, but leave them on so you know the exact size of your bed. Place the mattress on the box springs when you are finished tightening the bed frame.