The easiest way to flatten a whetstone is by using the flattener that often comes with it. Start flattening the whetstone by drawing a cross-pattern on the surface. Lubricate the whetstone and start rubbing the flattener from one end to the other. The grooves on these devices help move excess slurry. Check if it needs another round by flipping the whetstone on its side after all the patterns disappear.