Start by building the external frame. Toe-nail the boards together to create the long sides.

The 14' long board is placed between the two long sides, as shown in the close-up of the corner in the second image.

Once the external frame is built, measure the two long diagonals and adjust the frame until they are equal to square the assembly. Also make sure that the length from the existing foundation to the inner edge of the end is 26'.