How far out of whack is it?
First, be very careful to keep your cutting, stitching and piecing accurate. I know this because I didn't at first. After the quilt is finished lay it on a clean sheet on the carpet. Measure diagonally from upper corner to lower corner across the quilt to determine how much it is off. Spray the quilt with water until just a little damp. Stretch the quilt and pin with T pins. Continue until the diagonal measurements are the same. This takes quite a while. Allow quilt to dry completely. I just finished a quilt which I thought I had done accurately. However, one row was shorter than the other. I had to take the other rows apart and do them over. Good luck to you. Sometimes you just want to cry!
Sorry, the image didn't come through. (first post). I did upload it but it isn't linking correctly. Added the image as my avatar.