You need three things to see the northern lights:
- Darkness (winter rather than summer and as little light pollution as possible)
- A location at the right latitude (in the so-called “aurora belt” - it actually moves depending on the intensity of solar activity but the point is that further north is not always better, although I have seen great displays in Svalbard)
- Lack of cloud cover
This will maximize your chances to see the northern lights. Activity goes up and down and there are apps with short-term forecast but you cannot predict that a month in advance or find stable differences between places along the “aurora belt” so no need to agonize over which specific spot you are going to.
Perhaps a bit surprisingly, the most important factor on which your choice can have some influence is therefore the cloud cover (and not the aurora activity itself): check climate charts, avoid places on the coast, look at the weather forecast when you hit the road, etc.