I take a couple of passes on each corner. (I keep my plane set shallow). With practice you will now how much to take of each corner before going on. but my process is
1 - take a square and make it octagional (remove each corner)
2 - take the octagional, remove each 'corner' making it 16 sided,
3 - depending on the wood used, you can repeat and make it 32 sided or sand. Softer wood like ceder sands down real easy.