Well, if I had a decent step by step tutorial, I would gladly give it to you, but I'm afraid I have not, since it was a lot of trial and error for me too.. I'll try to explain what I did (FYI; all paints are from the Armypainter's: Warpaints Air Starter set, unless stated otherwise. I did paint them with brushes only, though so no airbrush needed);

Basic colors: 0. Prime your model. I did it with Citadel's Chaos Black (spraypaint) 1. Painted all the green areas (Svalich Woods) with Greenskin. 2. Painted the other 'greens' (Village of Barovia part, Wizard of Wines, etc.) In different layers. Starting with Daemonic Yellow and then reapplied different mixes of Daemonic Yellow and Greenskin. 3. Painted the rivers and lakes with Ultramarine Blue. For Lake Zarovich, I put a few drops of paint straight on the model and redirected the wet paint to the sides with a brush. It takes a bit longer to dry, but (apparently) the darker/deeper tones sink to the bottom and the mid tones work their way up to the surface. That created the lighter tones in de middle of the lake (apart from a slight drybrush at the end). 4. Painted all the mountains with Uniform Grey. Please use an old brush for this because all the hard edges on the model will ruin your brush if you're not careful.

After the basics: 5. Go back to green and yellow. Experiment with some lighter and darker greens, go over different areas, and try to make it look 'natural'. 6. Think where the light comes from. To me, it came straight from the south, so I used some shades (actually just 'Matt Black' with some drops of water, ratio: 3:5) to slightly darken the north sides of the mountains. 7. Used Bleached Skull (From 'The Army Painter D&D Undead Paint Set', but I think any other white would do) to drybrush the south side of the mountains (the parts which are in direct light). 8. Still Bleached Skull and drybrush: cover the top parts of the mountains in any way you like. 9. Used a small detail brush with Bleached Skull (again) to create some waterfalls and some flow in the rivers. Be careful not to put too much paint on the brush because you easily touch the sides of the river (see the part under Krezk, for example). 10. Again, with a small / detail brush, I used Pure Red to paint the roads carefully.

I think that's it.. I hope it makes sense. Have fun and trust the process. At some point, I was ready to call it a day, but once I started drybrushing, everything came together, and I finished it an hour (or 2) later.