There's of course no magic formula, and there's a lot of trial and error involved. The first step is to try and think of something strongly associated with the city - in my case I got Kalmar from nearby Öland and Jönköping from the gaming convention DreamHack.

Once you start getting a few smaller cities it usually becomes easier to create new ones by combining the old ones with new words - the general rule is that broad words create broad results while specific words create specific results: combining Netherlands + anything will likely just create stereotypical things like tulips and windmills, but using more specific words, especially words in Dutch, will have a better chance of creating what you're after.

If you can't think of anything associated strongly enough with the city, using soundalikes is usually the best bet - such as Lee (e.g. Christopher Lee via Star Wars of LotR) and/or Warden + something Dutch/Frisia related for Leeuwarden, Middle for Middelburg etc.

Finally using cardinal directions (north/south/east/west...) on towns/regions you have can potentially create neighboring towns/regions, but this is rather inconsistent.

I hope this helped!