Clean a plastic chair with an all-purpose cleaner or ammonia. Use a sponge or rag, and get into all the nooks and crannies. Rinse the chair well with water. Let it air-dry, or wipe down the surface with a clean rag. Primer or any other prep work is not required for plastic.
Shake a can of spray paint specially designed for plastic to mix the paint. Every minute or so, shake the can again to keep the paint well-mixed during the painting process.
Hold the can 10 to 12 inches away from the chair and apply a thin coat of paint in horizontal, sweeping motions. Apply a second coat after the first coat dries to the touch, usually in about 15 minutes.