Are you trying to make the flat sheet smaller so it is a queen size? If you are trying to reduce a king size

fitted sheet to a queen size sheet, you are going to have to reduce the size of the entire sheet and re-do all the folded and stitched corners...thats alot of work...You could just sew a long seam accross the bottom to shorten it, down at the bottom by the foot of the bed...but not involve the corners...and tuck the extra on the sides and bottom under the mattress.....or you could open the kingsize corners until its a flat sheet and tuck the whole thing in....Must be great sheet to go to this much effort...D.