Measure the location where the joist will be installed, using a tape measure, and cut the joist to the desired length, using a saw. Look down the joist holding one end to determine if it's straight, crooked, twisted or has a crown. Mark the side that has the crown if one exists, using a pencil.
Place the joist by positioning each end of the joist on top of a nearby wall. The crown must be up before you nail it in place.
Toenail the joist to the top plates of the walls. Drive the nails into the bottom of the joist at an angle that will allow the nail to pass through the joist and secure into the top plates of the wall.
Cut and nail in place blocks of wood between the new joist and the nearby joist that are installed parallel to the new joist. The blocks will be installed on both sides and at 90-degree angles near both ends of the joist. Blocks should be placed in the middle if the joist spans a long distance and more stability is desired.