I have a tendency for my cut line to waver. I normally resort to a mitre box where possible.
It takes practice
But they help fools like me manage to do some basic work.
The old chap who said that to me (although in all honesty I am now somewhat older than he was at the time) was bemoaning the loss of hand tool skills amongst the trades. TBH I think he had a point about the need to learn the basics with hand tools before "progressing" to power tools.
In recent years I've had a few apprentices attached to me for a few months each and they invariably eye up my little Makita 165mm cordless circular, possibly because it has almost completely replaced my handsaw, and maybe because with it I can produce smooth, straight, square cross cuts. But they only get to use it once they can do a half way decent cut with a handsaw consistently. I agree that you are in a different situation, but allow me to point out a few possible "gotchas" in what you may have done:
1. Odds on that the zero degree stop on your saw isn't actually zero, but that you set your saw to that 0 degree mark. Professional saws often have a grub screw to adjust the zero point, but it may surprise you to learn that out of 8 professional saws I've bought new in the last 12 years, I only recall two having an accurate 0 degree (plumb cut) setting out of the box - even one kf my two Festools was about half a degree off. So check and set that 0 degree cut with a square and then by making a test cut in a piece of something like 3 x 2 CLS, which is checked using an accurate square. If a saw ever gets dropped, as happens, the zero has to be chevked and reset before the next cut is done
2. Odds are that you have never checked that your square is square. This is the self-check that you can with the square against the edge of a sheet of plywood, MDF, etc. A surprising number of squares aren't square out of the box (the worst offenders seem to be cheap combi squares), and even if they are dropping one on a hard floor or throwing it into a tool box when you've done with it isn't helpful to accuracy. If your square isn't accurate you can't set your saw up accurately. If you don't have a reasonable combi square, take a look at the Bahco CS300. lt's no Rabone or Starrett, but it is a lot more robust and accurate than any of the sub-£10 POS combis I've seen apprentices turn up with
3. How are you guiding the saw? Often people try to follow a pencil line. It is far easier to slap a 7in speed square onto the wood and use that to guide the saw's base plate. That or make-up your own cross cutting guide
4. How good is the surface you are working from? Cups and bows across the width can play havoc with cut accuracy as the saw base plate tries to follow it. Same goes for very rough timber. If you have either it can sometimes be worth fixing a straight edged rip of ply to the offending surface, squared off the adjacent edge with packers under the ends or middle to level it up. You really don't need to do this all that often
