Add Support Between Posts
Lattice typically comes in 4' x 8' sheets. If the posts under your deck are more than 4' apart, you may want to install non-load-bearing posts at 4' intervals. Cut 2" by 2" supports to fit the length of the deck posts as well as the area along the bottom of the deck between the posts. Attach the supports using exterior-grade deck screws.
As an alternative, use H-molding to seam pieces of lattice together (Image 1). If you use H-molding, you'll need to install a stake at the bottom to hold the lattice in place.