Actually accomplish something and not just sit around shooting themselves in the foot for 2 years is step two.
I think a big problem is if they do win the House (and if they're miraculously lucky enough to win the Senate, too), they won't win with some people no matter what they do. Everyone who voted for them will immediately want them to start impeachment hearings against Trump, or will want them to try and block virtually anything Trump and the GOP wants, or things of that sort. And if they do that, that'll make them look like obstructionists to the right, and will only add to the chaos and gridlock and fuel the "See, both parties are bad" narrative in some people's eyes.
But if they don't fight back against Trump hard enough to satisfy their supporters, those voters will get frustrated and perhaps feel like nothing's going to change, and as a result, could feel demotivated.
So I honestly don't know what the best way for the Democrats to handle things relating to Trump and the GOP in general would be going forward, should they gain control. I don't want them to give in to Trump. I want them to do what they can to hold him accountable. But I also don't want people to see them as obstructionists. If there's a way they can do their jobs without risking alienating some aspect of the population, great, but I don't see that being likely.
I think, obviously, the biggest thing that could help give the Democrats a serious boost is if and when the Mueller investigation starts revealing everything they've gathered on Trump/his campaign and whatever role one or both of them played in the 2016 election mess. At that point, I think most Americans would agree the Democrats (and the Republicans, for that matter, if they're still in control of the Senate) would be fools to NOT immediately start hearings and focus on impeachment charges and things of that sort. That could perhaps embolden the Democrats to where they feel they've got enough public backing to do more of what they really want to do, and if they manage to hold Trump and any other people involved accountable, that'd be a great wave on which to ride into the 2020 elections.