Certainly!
We're gonna type up a tutorial, but I'll see if I can't give you a quick intro here:
There are two kinds of nodes, floors and walls. Representing floors and walls in DF (duh).
A simple floor entry might look like this:
<Floor spriteIndex = 6>
<!-- Boulder -->
<GameID value = 402/>
</Floor>
This means that floors which have the ID 402, will be drawn as image number 6 (remember, the first image is 0) from floors.png.
Adding more than one <GameID value = /> tag means that all floor matching ONE of them will be drawn as image number 6:
<Floor spriteIndex = 7>
<!-- plants -->
<GameID value = 231/>
<GameID value = 34/>
<GameID value = 24/>
</Floor>
All these floors will show up as image number 7.
Adding a <Material name=... /> tag makes exceptions based on soil layers.
For instance, all sand, clay, soil and dirt has the floor ID of 354. In order to draw a different image for sand or clay, you add Material conditions like the one seen below. Material conditions are ORed, so only one of them need to match.
<Floor spriteIndex = 25>
<!-- Soil floors -->
<GameID value = 354/>
<Material name="CLAY" />
</Floor>
Walls work the same way :)