Cover the floor in paper underlayment, rolling it out in slightly overlapping courses over the whole floor. Staple it down.
Mark 1/2 inch out from the wall where you want to start the flooring (usually the longest unobstructed side of the floor), putting marks at both ends.
Stretch your chalk snapline between the two marks and snap a line on the paper, as you have a chalk starting line that sits 1/2 inch out from the wall. Do the same thing on the opposite side of the floor.
Measure between the two lines, across the floor. Divide that measurement by the width of one of your floorboards, to figure out how many courses of boards you will need and how much space will be left over at the end. So if space is 196 inches wide, and your boards are 3 inches wide, then you can lay 65 boards and have 1 inch of space left at the end.
Add that figure for the leftover space to the width of a full board and divide in half. So a 3-inch-wide board, with 1 inch added to it, then divided in half, is 2 inches.
Use a table saw to cut the boards for the starting and ending courses along their lengths, so they are with correct width (2 inches, in this example). Cut from the grooved edge for the first course, then laying the flooring course by course, then cut from the tongue edge when you length-cut the boards of the final course.