The biggest mistake? Adding too much water too fast. You can always add more water, but you can’t take it out. Start with less, stir, and then add more if it looks like a lava flow instead of a sauce.
Also, do not skip the salt. Tomato paste is concentrated, but it’s not salted. If you don’t add salt, your sauce will taste flat, like a joke with no punchline. Add salt in stages, tasting as you go.
And whatever you do, resist the urge to add too much sugar. A tiny pinch balances acidity, but a spoonful turns your pasta into dessert. Nobody wants tomato candy.