In my experience, the only time they'll be perfectly flat is when they've been dry-mounted on card. To get them tolerably flat meantime, I follow much the same procedure as others above, but with much less equipment. Air dry for a few hours face-up on taut fly-screen material until the emulsion side is visibly dry. Then turn over and let them finish drying face-down. Once touch-dry, they will be curled along the edges. Place them in neat stacks of up to 6 sheets, all facing the same way, with a waste print to protect the emulsion of the top one. Place each stack on a flat surface with a pile of large heavy books on top. All the books must be larger than the print. Leave for 3-4 days, or longer if you can bear the suspense!