To measure the piece properly, use one of the tape measures. Also, if you are building a unit for which you will cut several pieces of the same length, be sure to use the same tape measure. Different tape measures can lead to slightly different results.
Position the clip on the end of the tape measure at the end of the board. Make sure it is hooked right at the end, and is not caught on a nail, or on one of the rivets holding the clip to the tape measure.
Pull the tape out a few inches beyond where you need, and lock it into place.
Once you find the correct measurement on the tape, make a small mark at that location, trying to be as precise as possible. Some people use a "V" shaped mark, with the point of the "V" being the proper location. Regardless, mark in such a way you know exactly where the proper mark is. Also make your mark noticeable enough so you can come back and find it again.
