Measure the desired dimensions of your mantel shelf, and mark in painter's tape.
Cut the pine lumber into pieces that will fit the dimensions of the mantel shelf. Use a leveler to make sure you'll install the pine lumber support at exactly 180 degrees.
Apply construction-grade, non-flammable adhesive to the the pine lumber panels if there are no studs in your wall. If there are, locate them and nail the pine lumber panels into the wall.
Nail your reclaimed or distressed rustic wood panels over the pine lumber, covering its dimensions exactly.
Use an especially wide piece of reclaimed wood as the "crown" or "cornice piece," otherwise known as the mantel shelf. Nail it into the pine lumber to create a shelf.
If the shelf doesn't feel sturdy, you can bolster it by cutting two 90 degree triangles and wedging them at each corner of the shelf, using construction-grade, non-flammable adhesive or nails to attach, depending on the brittleness of your wood.
If desired, apply a sealant or stain. Make sure whatever sealant or stain you use is non-flammable.