I don't know if this is the official way to do this, but it is what I do.
1) Sew left and right side sashes (strips) on the block. Your square is now a rectangle.
2) Sew the little squares on the top and bottom sashes. This should make the sash equal to the length of the rectangle you created in step 1.
3) Sew the longer sashes(strips) created in step 2 onto the top and bottom of the block. Make sure you match the corners with the left and right sashes.
Hope this helps!