A possible solution in a pinch is to get an already flat surface, ideally larger than your board. Cover it in something that will transfer (ink, paint, toner etc). Rub the face of the board you want to flatten across your flat surface**. The** transfer substance will pass onto the high spot in your board. Scrape, chissle or sand the high spots down slightly.

Repeat this until most of your board is marked by the transfer substance. Your board will be mostly flat (or at least as flat as the reference surface).

This technique is used in metal work, but it’s labour intensive. For woodwork to achieve sufficient flatness planes are quicker and produce a better surface finish. But if you don’t have any large ones, this method might work if your desperate and don’t want to buy new tools.

For a less accurate flatness. Place the board on the flat surface and push in the board to fell the points in contact with the flat surface. Then take those parts down.