As wood ages and dries out, it shrinks. Because of this, the joints between the parts of a chair will sometimes come loose. This can happen where the legs join the seat, where the side stretchers and cross stretchers join into each other, and where they join into the legs. These joints can easily be repaired.
Examine every joint and find the loose ones. All of these joints should be fixed at the same time.
Separate each joint as much as possible to make it accessible for cleaning.
Clean off all of the old glue from the ends of the legs or stretchers and from the insides of the holes. Use a knife, scraper or chisel if necessary. The new glue will not stick to the old glue.
Apply glue to both the tenon (the end of the leg or stretcher) and to the inside of the hole.
Put the parts back together using a rubber mallet or a clamp if necessary.
Hold the joint tightly together, and from the underside, drill a hole slightly smaller in diameter than your wood screw at an angle to the joint as shown.
Drive in a screw that is longer than the diameter of the tenon part you are repairing. This method is used in chair factories. The screw holds the joint until the glue dries and prevents it from loosening again.