Flip the chair over to find the screws underneath the seat cushion. Use a screwdriver to loosen the screws and remove the seat cushion. Keep the screws for later use.
Place the old seat (fabric, foam and batting removed) on a piece of sturdy medium density fiberboard, or MDF. Trace around the old seat with a pencil.
Cut along the lines with a jigsaw all the way around to create a new seat bottom.
Place the empty planter, upside down and centered, on top of the new MDF seat. Trace around the planter with a pencil.
Measure 1/2 inch inside the planter outline using a tape measure and draw another line following the original outline to create a cutting line. Carefully cut along the inside line with a jigsaw.
Secure the MDF seat to the chair by screwing the old screws into the original holes in the bottom of the chair and into the new seat.
Sand the entire surface of the chair and new seat lightly with 120-grit sandpaper. Sand only long enough to remove gloss and scuff the surface, rather than removing all of the original paint or stain. This gives the primer a better chance of adhering to the chair.
Wipe away all sanding dust with a tack cloth or clean, damp rag.
Place the chair outside on a large flattened cardboard box or drop cloth to protect the work area from paint.
Apply a coat of spray primer to the entire surface of the chair and seat using a long, sweeping, back-and-forth motion to ensure even coverage. Allow the primer to dry thoroughly, approximately one hour.
Spray the chair evenly with a thin coat of spray paint. Apply additional coats, within one hour of each previous coat to allow for adequate drying time between coats. Once the chair is covered sufficiently, allow the paint to dry for 48 hours -- or longer if the temperature is below 70 degrees.
Brush on one coat of polyurethane sealer to protect the paint and prolong the life of the chair, especially if placing it outdoors. Allow the sealer to dry for 24 hours.
Add soil and plants to the pot and insert it into the hole in the seat.