I starch, iron and trim all my blocks to the correct size. Then I lay all my blocks out the way I want them. Then chain sew collum 2 to collum 1, 3 to 2 ... and so on. All the blocks will be attached together. Then sew row 1 to 2, 2 to 3, till done. When all the blocks are sewn, iron the top. If you want to attach a border, measure both sides and middle, take the average, cut the border that length and attach to the sides. Then measure the ends and across the middle, average and cut the border for the ends and attach.
With careful measuring and sewing, the top should be nice and square.