I printed a robotic arm for school. Unfortunately, we weren't using a high quality enough printer, so the tolerances were off and things didn't slot together well. I'd recommend people to know the precision of their printer before setting off to build this.
Learning this is part of the journey imho. You don't even have to know that you have to pay attention to tolerances beforehand. You'll inevitably learn about it when building such thing.
Just always be aware that these things will never be perfect and don't get anxious because there are so many perfect looking projects on the internet. They most likely went through the same mistakes and might even have more people in the background. Just enjoy the journey
I tend to do this. I know I could get better at printing (though my printer is pretty old), but sand paper and a rotary tool are really fast and can be pretty precise and accurate too.
I'd PSA recommend anyone just add 0.25mm fit clearance to every single mating surfaces within their designs. It's not ISO or anything compliant - somewhere between Atrocious and Enormous range and perhaps an mechanical equivalent of Python code with no __main__, but just works for me, and it should for lots of purposes.
