One of the secrets to accurate work is in marking out your cut lines.
I was taught to use a square and a carpenters pencil when I first started.
However a marking knife is far more accurate and can also be used to form a cutting guide (or kerf) for the saw. This prevents tear out and makes for very accurate cutting.
A pencil leaves a mark around 1mm thick.
Whereas a marking knife is less than half that
A cutting knife can be left or right handed or both. The one above is bidirectional and has single bevel cutting edges on two planes. All have a flat back which assists in getting the edge against your square.
Next you will need a suitable square
Place the square on your work where the cut line is to be.
Score a line tight against the square.
The waste is to the left and the work is to the right.
You may need several passes to get a decent line in hardwood.
This severs the fibres on the face of the work and reduces the chance of tear out.
Now use a chisel to cut a bevel on the waste side. It only needs to be 1mm or so deep.
This provides a wall to guide the saw blade, which is known as a kerf. Its surprisingly effective!
Use a sharp cross cut saw to make the cut. This one is a frame saw with a 16tpi mitre saw blade. It makes short work of the cut, but still leaves a clean edge on the cut face.
Place the blade in the kerf you created and gently start the cut.
Once started the blade will follow the kerf with ease so long as your stance behind the blade is good.
This 1 1/2inch thick board took about 30 seconds to cut with this method.