If you don't have a lapping stone, get some very good sandpaper that won't embed grit in your stone; Keith at handamerican.com sells excellent stuff very cheap. If you're taking off a millimeter then you'll want around 300 grit. Lay it on a tile or anything you know is completely flat, wet the paper and do figure eights with the stone until the pits are out. When it's good and flat it will stick to the paper&tile with great suction. You can then use a finer (1000) grit sandpaper to smooth it, but oddly enough with the coticules this never seems necessary. Coarse-on-fine in this case seems to work great.

Don't forget to round/bevel the edges. Your bout seems to have some dimples at the edge, worth getting out or smoothing.