Hi Mia, are you looking for how to repair wood chairs?
Clean the old glue from the mortise and tenon. Then fill the mortise about halfway with epoxy. Set chair leg (tenon) into mortise, align and clamp. Some joints are just too damaged to allow for a tight glue joint, especially when repairing areas that have broken several times.
You would need to remove old glue and then apply wood glue and bind the chair joint so it doesn't move by tying it to the others to form a ridged frame until glue is dry - 24 hours.