Sharp blades, shallow cuts and circular motions with the plane to effectively make slicing cuts across the blade width helps and you can use alcohol on the knot which will soften it a little at the surface. This only works on solid knots and loose knots need to be glued in or removed and patched. Of course the problem with solid knots is threefold. Anyway that you approach the task you will be planning against the grain in part of the cut, end grain in part of the cut, and knots are HARD. Try duplicating the cutting action by “planning” a knot with a sharp chisel and the circular slicing motion advantage will be pretty obvious.