Attach strips of wood vertically to the sides of the posts that support your porch. If you want the lattice to be flush with the outside faces of the posts, set the strips back on the sides of the posts a distance equal to the thickness of the lattice.
Measure the height and width of the space beneath the porch and between two of the porch posts. Cut a lattice panel to that size, using the handsaw.
Set the piece of lattice into the space so it butts up against the strips of wood you attached to the sides of the posts.
Attach the lattice to the strips by nailing with small finish nails.
Repeat this process around the perimeter of the porch, filling in all of the spaces with pieces of lattice.