Working with wood requires careful measuring. Board sizes can change due to a number of factors so measure accurately every time, using the same tape measure throughout the project.
Never take dimensions and/or squareness for granted when working with boards and sheet goods.
Due to fast milling processes, shrinkage, and other factors, board ends may be out of square. Boards may also taper slightly along their length.
A 4x8-foot sheet of plywood usually has true edges, but check it for squareness and measure its thickness. A 3/4-inch-thick sheet may be slightly off -- a good reason to buy all the plywood you'll need from the same batch at the same time. If the sheets are not the precise thickness they're supposed to be, chances are they'll at least be uniform in thickness.
To measure accurately every time, use the same tape measure throughout the project. Furniture making requires precision; use a mechanical pencil, which makes a finer line. If you use a carpenter's pencil, mark with the narrow edge of the lead.
Always check a board's end for squareness before measuring and marking other cuts. Put the handle of a try square against the edge of the board with the try square's blade across the end. Light showing between them indicates the board is out of square. Mark the board and cut it square.
Tape measures become inaccurate if the rivet holes in the end get elongated. If that happens, line up the 1-inch mark with the squared end of the board, then subtract an inch from the tape reading when marking at the other end. Better yet, avoid confusion -- replace the tape. And use the same tape throughout a project.
Marking materials for ripping (cutting a board down its length with the grain or plywood in its long dimension) requires great accuracy. If you have a straightedge that's long enough, use it. If you have a table saw, you don't need a guideline to follow. Simply set the fence away from the blade to the width of the cut, then feed the material through.
For cut lines in boards or narrow rip cuts in plywood, many woodworkers use a marking gauge. Its thumbscrew-adjustable fence slides along one edge of the material while a sharp pin set in its post scribes the cut line. The post length determines maximum width.
If your project has many parts, a cutting diagram will keep waste to a minimum. It also helps you maximize the use of grain for appearance because you'll be able to see how the grain runs on each part.
Select which side of the material you want to show. Then follow the materials list provided with your project plan and lay out parts directly on the wood. Draw light, erasable pencil lines. Write a part number on a masking tape label for each part and attach it to the wood. You can use a dark marker for greater visibility because you won't have to erase pencil marks later. When marking cuts, make an allowance for the kerf -- the material removed by the blade as it cuts -- so your pieces aren't too small.