Each grand hex has a base terrain as shown on the kingdom-scale map. It will have secondary terrain from one or two of the adjacent grand hexes; each small hex will either match the base terrain, or one of the secondary terrains. We could use a d6 to select secondary terrain, but instead we'll use a d12 representing directions on a 12-hour clock face: 12 o'clock is north, 3 o'clock is east, 6 o'clock is south, and 9 o'clock is west. If the hex grain for grand hexes is vertical (columns are straight, rows zig-zag,) then even numbers are the hex sides, odd numbers are the hex corners and thus combine the terrain of two adjacent hexes, instead of just one. If the hex grain is horizontal, it's the other way around. Combined terrain works exactly like overlapping terrain blobs.
Before you even begin Step Four, you need to sketch the rudimentary rivers (from Step Two) and roads (from Step Three) indicated on the kingdom-scale map. Do this lightly, in pencil, because we'll add some twists to these paths later. Also mark the home base in the middle of the map and any other settlements that would be within the territory represented by the barony-scale map.
For Step Four, you will need:
2d12 of different colors (light and dark)The twelve-sided dice are used to define lines of secondary terrain that cross the grand hex. Each line begins at the clock position indicated, either a hex corner or the midpoint of a hex side, and extends towards the opposite corner or side. Each color of d12 is paired with an eight-sided die, which indicates the length of the line in hexes. The line is one hex wide.
2d8 of different colors (light and dark)
The secondary terrain is the same as the terrain of the grand hex it connects to, or the combined terrain of two adjacent grand hexes. If this terrain is the same as the base terrain, interpret the d12 result on the d10 terrain table (treat 10 as 9 and 11-12 as 0.)
The ten-sided die is treated as a terrain blob of a random variety, using the 2d4 to define its shape and size.
Roll all the dice at once for just the central grand hex (assuming that's where the home base is.) Roll again for any grand hex the PCs ask about or travel into.
If any of the routes (rivers or roads) drawn on the barony-scale map turns sharply instead of going straight, there's an obstacle (higher ground) that prevents it from going in the expected direction. Add this terrain to one or more small hexes on the map.