Start With Accurate Marks
To get an accurate cut, you have to start with an accurate mark. Stretch out your tape measure, place your pencil at the correct measurement and make two marks that form a “V,” with the tip of the “V” pointing at the exact measurement. A “V” is more accurate than a single line, which can stray slightly to the right or left and throw off your cut mark.
Using a square, mark your cutting line over the tip of the “V.” Finally, put an “X” on the “scrap” side of the board; that's the side of the line you want to cut along. Cutting on the wrong side of your line can make a 1/8-in. difference in the length of your board; sometimes this is a big deal, sometimes not.