The lines will guide in cutting and nailing pine boards. Then, use a small handsaw to cut the door casings to the scribed lines. Finished floor slides under trimmed casings. This line ensures that the starting row of boards will be straight. After marking board length so end aligns with joist centerline, cut with a circular saw.
Square helps guide saw.
Place first board on layout line and secure it with 8d or 10d cut nails. Use three nails per joist for 1 x 10 or 1 x 12 boards. End joints between boards are located on joist centerline. Drive cut nails at a slight angle to be sure they enter the joist. To straighten a bowed board, drive a wedge between board and scrap block temporarily screwed to subfloor.