The only way i could ever make it work is to mark all four sides and plane at an angle to each line. Going lightly to stop at or before it a bit.
Then when all edges are to the line go across or diagonally keeping the plane level.
Mark the board with pencil lines every few inches and if you plane a line off on one of the slopes remark it.
The hardest part for me is to bAck off the iron and sneak up on flat with the plane.
Go slowly and watch the lines. They get shorter and shorter