It is up to you to determine how wide each strip is. I like mine @2" for a 12" block. So the width of the strip would need to be cut at 2 1/2" - I usually draw my grid on paper so I can count how many sashings and how many corner stones it will take. You also have to decide how you will treat the outer edge: Are you adding the sashing color as a border or are you extending the corner stones to the edge. Either works - I've done both.
Assuming you want to put a solid border around the outside you will need 8 strips 2.5 x 12.5 for the horizontal pieces. Sew the strips to the sides of your center block and attach the sides. That gives you the width of a row.
Then you will need 9 strips 2.5 x 12.5 for between the rows. You will also need 6 corner stones 2.5 x 2.5
Using the same seam allowance, sew long strip to corner stone to long strip to corner stone to long strip.
I make sure that I press the side sashing on the rows opposite to the corner stones. That way they will all nest nicely when I attach the two together. I hope that helps.
When the top is sewn you can measure across the center to get the correct measurements for the border.