Cut an upright out of a 1-by-4 or 2-by-4 piece of lumber. If you build the brace out of 2-by-4 lumber cut the upright to 1 inch less than the distance from the floor to ceiling. If using 1-by-4 lumber, the upright can be the same length as the height of the wall.
Cut a cross member for the top of the deadman brace. This piece should be at least 3 feet long but no longer than 4 feet.
Fasten the center of the cross member to the top of the upright using 16d nails for 2-by-4 lumber or 8d nails for 1-by-4 lumber. Set the nails deep enough so no portion of the head extends above the wood. This prevents scratches to drywall during installation.
Cut two braces to fit from 1 foot down the upright to the cross member 1 foot from the center connection. Fasten these braces in place with the same nails used to fasten the cross member to the upright. The completed deadman brace should measure about 1/2 to 3/4 inch longer than the height of the walls of the room.