Press each and every seam. On the charm rows press the seams towards the sashing rectangles and on the cornerstone rows press the seams towards the cornerstones. Alternating the direction you press the seams will help with the alignment of the rows in the next stage.
Take a cornerstone row and sew it to the top your A row. As you get to each seam make sure the underneath seam is pointing in the opposite direction, if necessary stop about 1/2" before the seam, make sure the needle is down (in the fabric) and then lift the presser foot and use your finger or a small tool to sweep the seam in the correct direction. Then put the presser foot down again before you continue sewing.
Repeat with your rows B to G. You will have one cornerstone row left over - this goes onto the bottom of row G.
Now sew your rows together in the correct order until they are all assembled into one piece. Now press all your seams - you can alternate the direction or have them all pointing the same way - it is entirely your choice.
Lay your completed quilt top out and admire it - you have completed the first stage! Well done!