Press your top as flat as possible.
Measure the width in 3 different places, keeping away from the edges.
Write down each measurement and add them together then divide by 3, that is your average and that will be the size border to cut.
Repeat this same step for the length of the quilt.
Pin from the center out.
Cut length of fabric when possible, this helps avoid the dreaded bias.
Starch is also your friend.