I make sure my quilt top is well pressed and sometimes starched before measuring. I measure the shortest sides first(usually top and bottom) if they are the same, I cut my 2 end borders but leave an extra inch. I mark where the end should be in the seam allowance (Frixion pen)I pin to the mark and place pins at intervals along the length easing in any extra fabric from the top where needed. If the top extends over the mark a tad after sewing, I don't worry about and then trim off the extra. I follow the same process for the long sides.