As a rule of thumb, I've found half a turn of the spoke key on each of the spokes you want to move the rim towards is a good place to start when truing up a knackered wheel. Then keep on with half or quarter turns until the wheel runs true-ish.
Last step is to pinch the spokes together where they cross to get a rough guesstimate of the tension around the wheel - if you have an opposing pair that are both loose they can both be tightened a little without moving the rim.
First time takes a little while but once you've done a few it's surprisingly easy and a really useful skill (I've helped friends out and they've been dead impressed). Take it carefully and the worst case is that a spoke will ping when you tighten it, in which case it's a toss up whether to bother sourcing a new spoke or just look for a cheap 2nd hand wheel.
When I've trued wheels up that have been in a baaaad state I've sometimes found there is a little bit of "hop" left, where the rim runs all in one plane (so the brake blocks have a constant clearance), but the wheel is not quite round. In bad cases, if you hold the bike up and spin the wheel fast, you can actually feel the hop of the wheel. But I've ridden wheels in this state on three different bikes and they've always felt fine when riding, so I wouldn't sweat it if this happens to you too.