Lay one 3 1/2" square on the rectangle and sew on the diagonal. Trim the excess away and press the triangle to the outside corner. Lay the remaining 3 1/2" square right sides together on the opposite end of the rectangle. The second square will overlap the first seam and that is correct. Sew on the diagonal. Trim the excess fabric to 1/4" and press.
That is just one way to make Flying Geese.
Hope this helps.