What is the best way to attach ready-made 4' x 8' redwood lattice
panels to the horizontal redwood rafters of my patio cover? Do I use
nails or
screws? Do I attach the panels to the rafters tightly or loosely (so
the panels can expand/contract during the year)?

To preserve the panels, I'd like to apply paint, stain or sealer
before I install them. Which finish is best? Should I apply the
finish with a brush, a roller a sprayer? Or it is better to soak the
panels in the finish then brush off the excess?