though it is better to square after each step, there is another way to correct the out of square/size problem even at the end of the block. Mark on a piece of freezer paper the correct size and shape of the block. place that on the ironing board. lay your block on top of this and carefully pin the block with straight pins matching the correct size lines on the paper. use plenty of pins. When pinned down, Spritz it with water. this will relax the fiber and either stretch or shrink it a little to fit the pinned size. if it needs to shrink a little more hold a hot iron above it to heat the fabric. Leave the block pinned in place until completely dry.