I used more or less the same technique as for my Six Duchies & Cursed Shores map:
I scanned a tracing paper copy of the maps found in my paperback edition of Robin Hobb's Soldier Son trilogies and used it as base layer (on top with Multiply as blending mode).
On another layer I drew the shape of the land with a hard opaque brush in a bright colour. When this was done I blocked the transparency, filled with green (after filling another layer below with sea-blue) and then started adding different hue variations with a big soft brush with low opacity. I then decided where my light source would be and started working on the details, switching between lighter and darker shades (same for water).
I used several other layers for trees (I created a custom brush which I'm not too happy about), names and roads, and also another one between sea and land to adjust the colour of things such as sandy shores, lakes and rivers.
If I remember correctly, I also added a light paper texture to the whole.
Again, sorry, I don't have any WIPs of it.