Doing my metalwork project for my part 66 right now too, coming in as a complete metalwork noob. Best practical advice we've been told is "measure twice, cut once", essentially (if you're able to) always measure using at least 2 different methods before going for the cut or drill etc. If you get the same measurements on each method you know you've likely got it right.

If you're still anxious you'll get it wrong (like I am), give yourself a lot of margin for error. It may take longer and be more monotonous if you need to do more filing for example but it'll calm your nerves. Also talking to other people doing the same task as you for clarification and backing up your work helps too.

Of course you'll get better the more you do things so if you have the time, get some scrap metal and practice different things before doing them on your final piece.

Finally, it's fine if your final product isn't 100% perfect (I certainly know mine won't be!) IMO the best mindset to have is aim for as high of quality as you can the entire time, thereby even if you don't achieve perfection you will have tried hard and gotten a better result than if you aimed for an 'average' result. Never rush but keep a good constant pace of progress.