I measure down the center and pin the border (cut to that dimension) first on each end, then in the middle, then in half again, then in half again......If there is any discrepancy between the border and the outer side of the top, the difference will be absorbed or adjusted for evenly. If I don't pin, it has a tendency to get wonky. Once the sides are on, I press the seam and repeat the step for the other side.