Find the wall studs with an electronic stud finder. Mark the location of the studs with a pencil.
Acclimate the wood to your home's environment. Let the wood panels sit in your home for 48 hours before working with them.
Clean off the wood. Reclaimed wood may not be clean, or it may have insects. Use a wood cleaner and a rag. Scrub off any surface dirt. Use the end of a chisel to remove stubborn dirt.
Sand down splintered areas. Reclaimed wood may not be in good shape. While the rustic look may be desired, actual splinters can cause injury. Sand these smooth.
Measure out the length and width of the wall to determine how many panels you will need.
Cut the wood panels to fit with a circular saw.
Nail up the panels with a nail gun. Lay the boards horizontally across your wall. Drive in a nail through the panels into the studs. Continue until the entire wall is done.
Insert a pry bar behind the old mantel to pry it free from the wall. Be careful not to splinter the wood -- you can recycle it.
Remove any old nails that may remain in the wall studs with the back of a hammer.
Chisel off the dirt or debris from the reclaimed wood beam.
Nail up the reclaimed wood beam to the wall studs. Use the old nail holes from the former mantel as your guide.