Copy one of my lists.

Or, fly a list with ships or upgrades that you've never played or practised with; studied their dials to see what limitations they have or watched youtube X-Wing videos about.

Cheers

Baaa

put upgrades that have no effect or a worsening effect. (example engine Upgrade on A-wing or Darth Vader on a TIE-Phantom)

Upgrades are in the game for good now (unlike, say, the early Wave 3 metagame), and that's great, but you still need to know why you're putting that upgrade there. If you can't answer that question, you'd be better off skipping the upgrade.

On a more general level than specific mistakes, though, I'd say the best way to build a bad list is to build any list without knowing how it's going to win. Picture how it's going to perform against some lists you know it might face, whether that's based on tournament results or seeing your regular opponents play or even your own preferences. Ask yourself, realistically, what your best options for deployment are, how it's going to make it through the muddled, collision-prone mid game, and how it's going to close out a difficult opponent.

It's okay to get those answers wrong, of course. That's just gaining experience, and that's fine. But again, I find if I can't answer those questions at all, that's a good indication that the list isn't fully cooked yet, and I need to clear off my drawing board and start again.

Whelp. I've got no need to say anything now.

There are lots of good combinations at this point, and many ships even have multiple good builds (Paging Wave 2!), but if I'm not sure why I'm putting any given ship into a list, and what strengths and weaknesses it brings, then either I shouldn't be flying that ship, or the point of my list is learning how to fly that ship.

I play lots of "learning how to fly a ship" games.