I stumbled upon this technique on YouTube. You draw with water-soluble ink and then go over it with a water brush. This is what the drawing looked before I applied water:
I unearthed a cheap fountain pen and bog standard fountain pen ink from my old school supplies for this. I was surprised how much I like drawing with that pen, so much so that it might replace my mechanical pens when they run out of ink.
You have to get used to the way the ink reacts to the water (which will vary among different ink brands) — for example I didn’t expect the ink to turn that dark — but I’m happy how my first attempt turned out. It’s a fun process in any case.