This is how I see it: A1 is your first piece. Add A3. Then add A2, A4 and A5. Stop at this point. Side triangles are pieced: A6 to A9, then add A10 and A8. Sew to the A1,2,3,4,5 piece. Repeat with other side: A7 and A12, then A13 and A11. Sew to other side of A 1,2,3,4,5. This completes the block.
Hope this helps.