Once all your strips are sewn together and bagged it's time to sew the strips together.

Take strips number 1 and 2 out of their bags.

With the right sides up, line up the strips. Refer back to the picture of your layout to make sure that the strips are properly aligned.  

Pin the strips together with their wrong sides together. Be thorough with the pins and make sure to secure the corners well.

After they are pinned all the way together you can start sewing.

The strips are sewn just like the squares were sewn together earlier. Remember that you're sewing on the right side of the fabric.

When you get to a corner, move your needle  diagonally from the front square to the back square and then keep going as shown in the pictures above.

When you get the end of your thread tie a knot on the wrong side of the seam and hide the thread end, just like you did when sewing the squares together.

Sew all the strips together in this manner. The easiest way to do it is to sew Strips 1-5 together, then separately Strips 6-10 and Strips 11- 15. Then sew the 1-5 chunk to the 6-10 chunk and lastly sew on the 11-15 chunk.