Good craftsmanship in any material begins with careful and accurate measuring and marking. In woodworking, it doesn't matter how careful and consistent you are at cutting "right on the line" if the line itself is not precisely where it ought to be.
There are certain tricks and techniques to using any measuring tool for maximum accuracy. The first is to avoid parallax errors. If you do not view the markings from exactly the same angle each time you transfer a measurement to a piece, the measurements will vary.
This is why some steel rules have beveled edges, which bring their graduation marks closer to the work-piece. If your rule does not have beveled edges, you can avoid parallax errors by standing the rule on edge so the graduation marks come into direct contact with the piece.
If you must make several measurements to the same point on a ruler or yardstick, putting a piece of masking tape at the correct graduation can keep you from measuring carefully to the wrong point. Tape works better than a pencil or ink mark on the rule because it does not leave confusing lines.
To make your mark use a pencil with medium-hard lead such as 2H. A thin, straight line with a sharp point is more accurate than a broad, irregular line.
The way you hold most measuring tools is also important for accuracy. Do not hold a pencil perpendicular to the work surface. Angle it into the corner formed by the work-piece and the edge of the rule so the line is drawn right on the edge rather than held off from it.
When making very long lines, have a helper hold the rule securely. You can also keep the ruler from slipping by clamping it to the work-piece with small C-clamps. Use bits of cardboard or scrap wood to keep the clamps from marring the work-piece.
When drawing (called striking) a line from a point marked on a work-piece, hold the pencil point on the measured mark and carefully slide the T-square or bevel up to the pencil point. Keep your pencil point sharp. You can make a chisel-shaped edge, which wears better than a point, by rubbing the pencil on fine sandpaper.
Some jobs require greater accuracy than is possible with a pencil. Laying out lines for tight-fitting joints requires the accuracy you get by using a utility knife with a sharp blade or an artist's knife.
Blades produce very fine lines that can also help start the cutting tool precisely. The cut wood fibers can help prevent splintering and ragged edges on an important cut. An awl does the same kind of a job when you are marking points.
Lay out circles or arcs using a compass or trammel points (these use points that slide on a steel bar and are used like a very large pair of dividers) depending on the size of the circle required. A caliper rule measures dowel diameters and small part widths more accurately than a ruler.