The humble pencil line on wood is not just a mark, it is heavy with consequence. Once the saw is introduced to the board, that piece of wood will never be the same size as it was before. For this reason, the measuring and marking phase can use your undivided focus, especially if it is your first time working with wood. Even if your end product is just a small shelf, or a little box or tray for a craft table, it is worth taking some time to see the measuring tape, pencil, straightedge, and try square as more than just a way to get you to the cutting phase.
Take a look at the board and its edges. Is there an obviously warped area, or any other irregularities? Is there any chance that it will throw your measurements off? For example, if your end is not square and you are not careful about how you set the tape measure, your cut could turn out slightly shorter (or longer!) than planned. Pick one edge that you want your cuts to run to/from, and stick to using that same edge for related measurements. If you change your mind on the same piece and end up using a different edge to reference the other side of your measurements, you can end up with subtle discrepancies that can become glaring later on when you have to match up multiple pieces.
Try and avoid drawing a really wide or vague pencil line. If your line is too wide, it can be difficult to know exactly where the saw should be placed. You also want to use a sharp pencil and a guide of some sort for accuracy. The type of saw that you plan to use to make the cut and the type of line you need to draw will guide what the best tool is to use when marking the wood. Make sure the pencil mark is thin, dark, and sharp, and use a try square or straightedge if needed to make sure your line is at a good angle. Also, decide what your “waste side” of the cut will be by marking it, because many new woodworkers end up cutting the wrong side of the wood and ending up with a smaller product than they had hoped for.
One thing to try as practice is to make the same mark twice without making a cut, then make it again, etc. Just see if you can hit your line over and over. If you don’t get the same place every time, you should pause and ask yourself what’s changed between each time you draw the line, before erasing all of the lines and starting again. Did you not pull the measuring tape all the way to its end? Was the hook of the tape not pressed firmly against the edge? Was the board moved or did you shift position between measurements? You can learn more by slowing down, and seeing what’s causing the change, then taking corrective action, rather than making a hasty and potentially disastrous cut.
It is also helpful to support the board while marking or making a cut. If the piece you are cutting shifts while you are making a mark on it, the line can end up being a bit off from where you intended. The cut can also end up being off if the board moves while you are sawing. Use clamps to hold down the wood as needed, and make sure the board is sitting flat and is not likely to slide as you work. Again, this is not just about the safety aspect of clamping (which is very important). It is also about not undoing all the work you just put into getting your measurements and markings right.
When there is a set of parts that need to be the same length as one another, you can often save time and be more consistent by measuring out each part from the one part you already know is exactly correct, rather than having to make the same measurement over and over again for each piece. For instance, if you have a simple table that requires two side pieces that need to be the same length, you might be able to get more consistent results by marking out the side on one of the pieces first, making sure it is perfect, then using that piece to mark out the second piece. You can always make practice cuts in scrap wood first if you’re not sure. For instance, if you are measuring out two shorter pieces that are intended to be the same length, go ahead and make a couple of practice pieces, mark them out, and see how well they match after you cut them. If they are off, go over your markings, where you marked the waste side, where the saw is positioned, etc., before blaming your tape or tool for any inaccuracies.
A clean line doesn’t necessarily mean a clean cut, but it does make it much more likely that you will end up with a perfect cut. With time and experience, you’ll probably end up taking all those checks for granted. Is your measuring consistent? Your reference edge is the same? Is your line straight? The waste side is marked? The board clamped or held? Is your piece of wood positioned so that it’s clear where it will end up after the cut? When you can answer yes to that last question before you start cutting, your first cut is already in your favor.