Measuring and marking are basic to all other carpentry skills. They don't require complicated or expensive equipment, just concentration.
If you don't already own a quality tape measure, buy one. Get one with a 25-foot-long blade that's 1 inch wide and has a blade lock to keep the blade extended. The wide blade extends a long way without buckling.
If possible get a tape measure with the first few inches not divided into 32nds. These fine markings are rarely needed in a carpentry project and can be distracting. Learn to spot the 1/4- and 1/8-inch markings instinctively. They're good benchmarks for other measurements.
If you're a bit rusty, practice marking and cutting scrap. That way you won't ruin an expensive piece of lumber. And even though you've drawn precise plans, take on-site measurements. Your plans may be perfect, but the work site won't be. What matters is that the boards fit where they're supposed to go, not whether they're cut to the precise size on your plan.
To make an outside measurement, hook the tape over the edge of one board and read the measurement at the outside edge of the other.
To take an inside measurement, push the hook against the inside edge of one board and read the measurement at the inside edge of the other. You can bring the back of the case up to the opposite side of an inside measurement, then add the length of the case to the tape reading. This is less accurate, especially if the back of the case is not square.
Always keep the tape straight and at a right angle to the surfaces you're measuring. Tapes don't always agree, so your measurements may not match. Two tape measures, especially if made by different manufacturers, can vary by as much as 1/8 inch from one another. To keep all your measurements consistently accurate, always use one tape on a project.
Before cutting a board make sure the end that you won't cut is square. If it's not, cut it square and remeasure. Hook the tape measure on the end of the board and extend the tape until you reach the length for the crosscut. Mark the measurement with a V, not a straight line.
Hold the pencil tip on the point of the V, and slide a layout square over until it touches the pencil tip. Mark a straight line, then draw a large X to indicate the waste side of the cut.
Mark the width of the cut on both ends with a tape measure or square. Hook a chalk line tightly on the marks and snap the line.
If the cut runs parallel to the edge of the board, scribe a line by holding a pencil against a square at the proper width. Draw the square and pencil down the length of the board.
Miter cuts are angled crosscuts most often at 45 degrees. For a 45-degree cut, measure to the long end of the miter, and set your combination square or layout square on the mark. Draw the cut line.
To mark angles other than 90 and 45 degrees, use a sliding bevel gauge, or T-bevel. Set the blade and handle on the inside or outside edges of the boards and lock the blade. Then move the tool to the piece you want to cut, and mark the line along the blade.
Mark actual length on-site whenever possible. No matter how accurately you take measurements with a tape measure, there's no substitute for an on-site measurement. Any number of factors can throw a measurement off, and even a small discrepancy can make a difference. Lumber dimensions may not be exact—the actual dimension may be off by 1/32 inch, which your tape may not show, or you may forget which side of a marked line to cut. Position the stock to be cut as shown—with the cut end as close to the edge as possible, without slipping off—and mark the cut line with a carpenter's pencil.