The best way to accurately measure pieces of wood is by using quality measuring tools. The way to take a measurement varies and taking an inside measurement is different than taking an outside measurement.
The most important part of accurately measuring wood is to purchase a quality tape measure or folding ruler. Find one that fits comfortably in the hand and is not too long. Choose the smallest measuring tool for the job.
To take accurate inside measurements, use a folding rule with a brass extension. Mark the wood directly from the rule, adding length for the body of the ruler.
When transferring measurements to the wood's surface, press on the edge of the tape and mark the measurement using a "V." This makes it highly visible on the wood's surface. Cut where the two ends of the "V" meet, at the tip.
Always mark the spot with a very sharp pencil. A mechanical pencil also works well for measuring where to cut wood.
When measuring more than one piece of wood, cut one piece to size and mark it with the word "sample." Use the sample to measure each subsequent piece. Mark each piece with the pencil and cut.