We used to squugee them flat, with the image facing outwards, on a clean piece of glass, like a window.
They drop off when perfectly dry, so make sure they fall onto something clean. Naturally there is still a slight curl towards the image side, but no buckles or bumps. Ferrotyping is great if the plate is perfect and you like the mirror gloss- still beats the mirror gloss of any RC paper, but the nicest in unglazed glossy, as favoured by the great Ansell Adams, and many of his contemporaries.
Otherwise, squeegweeing off the excessw and then laying flat onstretched flyscreen material, (fibreglass, of course, not metal, and they dry naturally, but need to be flattened. This is best done against blotting paper in a dry-mounting press. This is the best use for such a press, as actually using it for mounting somehow reduces the beauty of a naked FB print.