Hi Alyce,
Are you wanting to miter the corners? What I would do is lay the block that is already made down on blank paper. Trace around it, then draw a square that equals the twelve inch wanted, add 1/2 inch, then cut out the original block from the center, measure the remaining edges, cut the fabric to either miter or block finish. It's easier for a new quilter, I think, eventually you get to the point of doing the math without having to draw it out, but for real accuracy......draw, measure each side, measure again each side, add 1/4 inch on each side(1/2 inch for sewing together) then cut and sew. At least this is the way I taught myself to figure the sizes I wanted. (self teaching anything can be hazardous to your finished project)
Does that help, Sug?
Sharon