Okay, I prefer to use the term spirit seals instead of sigil to prevent confusion with the Chaos variety so, to break it down for you:
A traditional seal for a spirit needs to be at least a reasonable imitation of what you are copying. It doesn’t have to be perfect, or flawless, but it does need to at least resemble the original in order to make a connection with the spirit. It is often said that you should draw it by hand in order to infuse it with your personal energy which helps to begin to establish that connection, so if you cheat and print it out, you should then go over the printed copy by hand (note: printed or digital copies of a seal will work just fine if you are too embarrassed about your drawing skills).
Now, when you talk about using a square to create a sigil, I am assuming you are talking about using something like a planetary kamea, right?
In this case, you break down the name using Grematia or numerology and trace it out on the square using straight lines. You mark the first with a small circle and then draw a straight line to the next, and the next, and then, on the last number, you mark it with a small vertical line signifying its end.
Using a “sigil generator,” like the Rose Cross method, is the same. You mark the first letter of the name with a small circle, and then connect each letter with a straight line, and then mark the last letter with a smaller vertical line. If you see a spirit seal that looks like this, it was drawn on either a square or the Rose Cross generally.
As for Spare style Chaos sigils…they can be as sloppy as you want them to be! The only one the sigil has to appeal to is you.