Disassemble the seat from the rest of the chair by removing any screws holding it in place. If the chair has existing webbing, use a router to remove it. Work carefully to ensure you do not damage the chair's frame.
Soak a sheet of rattan webbing in a bath of water for 20 minutes. The sheet must be large enough to cover the hole in the chair’s seat. When the rattan is thoroughly soaked, remove it and place it over the hole in the seat with the shiny side facing toward you.
Push the webbing into the groove using a wooden wedge. Work your way around the perimeter of the chair until the webbing is fully secured.
Cut off the excess webbing using a utility knife. Carefully cut off the edges so that no more than a millimeter of webbing extends past the outer edge of the groove. Work carefully to avoid cutting yourself or the newly installed webbing.
Apply a thin line of PVA glue directly into the groove on top of the webbing and let it dry for 5 minutes.
Insert one end of the cane spline into the groove of the chair and hammer it into place. Work your way around the perimeter of the seat, hammering the spline into the groove. When you reach a corner, dampen the spline with a wet cloth or sponge to make it more flexible and prevent it from breaking. Once you get back to the starting point, cut the end of the spline and hammer it into place.
Wipe off the excess glue and let the chair seat sit and dry naturally for about 24 hours. Once dry, reassemble the bottom into the frame.