Texturing. The not so fun stuff.
Adding textures to a maps is not a precise science and can be done in many ways. For the best examples open any of the maps used in the game to see how LHS put them together. The following is just a basic example.
The first thing we need is a base texture to be defined. Click on the "Edit Texturing" button in the top menu which will bring up a number of dialog boxes. In the "Edit Texturing" box, click on the "Add Layer" button and in the drop down menu, select "Mud" and then click on the "Set" button to choose the texture to use.

In the dialog box which pops up, browse to the Aztec folder and select "t_aztec_is_gd01_baked.dds". There are other suitable textures available in the other folders. I just chose this one as an example.

Now choose a solid brush set the strength at medium, radius at large and paint the entire landscape, except for what you want to be rock. When you're done, you'll notice that large blocks of texture will change colour. Don't worry about that. They will stabilize when a second texture is added on top.
Ok, since we want our mountains to look like rock instead, create new layer named "Rock" and choose t_generic_is_mountainsharp01_baked.dds from the Generic folder. Reduce the brush size and paint the mountains and cliffs with it. You do it roughly now. Later on you can touch up the edges as you zoom in and start placing objects. But we've still got those ugly blocks.

The first two layers placed were solid. Now we're going to add a vegetation layer on top of the first two layers. Create another layer called Grass. Then choose t_generic_is_foliage01_dif.dds for the texture. Unlike the other two textures which were solid this one isn't and can go over top of a solid texture. Reduce the brush strength to about 6% and lightly paint the rocks and mud with it. As you're doing so you'll notice that the texture blocks will all turn the same colour. Great, however you will notice that the grass if layed down heavy enough isn't as green as it should be.
For this check the Colour button in the Edit Texturing dialog box, click on the box next to it and choose a deep green colour. With this, paint over any area where you had placed the grass layer. This colour will only apply to that texture. Now that the textures are in place you'll notice a number of sharp shadows. Use the smooth tool to soften those edges later.
What's a resort Isle without a beach.
Use the Micro-Raise/Micro-Lower tool to create an apron of light blue where the beach will be. See above pic. Now create another layer called "Sand. For this I used genric_is_sand02_dif.dds. Use this to fill in the area under the water as well as up the land so that it looks like a beach. Next you can select the colour tool and paint the section under the water with a few patches of blue and aqua

This is only rough. As you're placing the towns and other objects you'll get in closer and see that you'll need adjust where there textures meet in quite a few places.
There are many ways to do this and combinations of textures which can be used. The best thing to do is to experiment until you get the look you want. What I used and how I did it are just an example.