A pencil line is wide. On fine work the difference between cutting on one side of that line or the other is the difference between a joint that closes and one that does not, and no amount of careful sawing recovers a line that was vague to begin with.
Use a knife where it matters
A marking knife leaves a line with no width and a small groove that a chisel or saw can register against. It also severs the surface fibres, so the cut edge comes out cleaner. A pencil is fine for rough carpentry and for anything you will cut generously and trim later.
Mark the waste side, not just the line, as soon as you have made it. Cutting the wrong side of a correct line is the most common and most annoying error there is.
Avoid measuring twice
Every measurement introduces a small error, and measuring each of six pieces separately gives you six different lengths. Where parts must match, set one and mark the rest from it directly, or use a stop so every piece is cut against the same reference. For repeated lengths, cut one and use it as the gauge.
Where something needs to fit an existing space, hold the piece against the space and mark it rather than measuring the gap and transferring the number. Direct comparison has no reading error in it at all.
Check your square
A square that is not square ruins work silently. Test it by marking a line across a board with the square held one way, then flipping the square over and marking from the same point again. Two lines that diverge mean the square is out, and half the divergence is the error.
Do this with a new square as well as an old one. Inexpensive squares are frequently out of true from the factory, which is the main reason to spend money on marking tools.
Read a tape honestly
The hook on a tape measure is deliberately loose, so it slides by exactly the thickness of the hook to read correctly whether pushed against a face or hooked over an edge. A bent hook therefore puts every measurement out. Check it against a rule occasionally and replace the tape when the hook is damaged.
Reading the tape at an angle introduces error too, and on a long measurement a sagging tape reads short. For anything over a couple of metres where precision matters, use a rigid rule or a story stick.
Mark face and reference edges
Choose one face and one edge on each piece as the references, mark them, and take every measurement from those. Timber is never perfectly square or parallel, so measuring from whichever side happens to be nearest produces work that does not line up, for reasons that are very hard to find afterwards.