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.
Sharper Chalk Lines
The fastest way to make straight cutting lines on plywood is to use a chalk line. But before you lay the line on the plywood, give it a quick midair twang. That first twang will get rid of excess chalk, and your mark will be less fuzzy and easier to follow. This is an especially important step to do right after filling your chalk line.