For added best results, use the same methods used by bowyers in reverse.
Wet the backside (non-playfield surface) with a sponge brush, let the wood absorb the water, and use a wood scrap frame to incrementally increase pressure with thumbscrews while keeping the edges braced across the playfield. You want a "frame" not single pieces of beams. With a frame you can also add small amounts of weights (equally) to speed up the process.
Do not oversaturate the wood, you do not want it wet completely soaked, as this weakens wood fibers excessively. Gradual heat is applied in drying room. You may need to repeat the wetting process several times before the playfield is properly flat. Gauge with a level via frame release once a week.
Severity of warping determines length of bracing and time in the frame.
Minor warping will correct itself in time without intervention or requiring this technique.
Putting the the playfield in the sun is not recommended for extended periods will cause cracks and wood planking.
You cannot properly control temperature changes through the day and evening, or humidity.