First you need some kind wood, metal or plasticard to make the round bases. These you need to cut using a cutter or saw. I made mine using a thin wooden board and sawing this at work with an bandsaw. You can do these as big as you want but I find them 10 cm in diameter to be a good size. Then I put some modelling material which is an air hardening clay, this particular one is really cheap and not very good but it's doing it's job good enough.
I then pushed this on to base with my hands and some water, unfortunately this time some parts fell off and I needed do redo this step one more time. As you can see on the picture I also made some wheel tracks using the Mortar model.
I used a model with it's base to mark up where I wanted the empty spaces to put the crew in. The clay I used is so fragile and so I needed to use a hobby knife to help cut up these square holes. I also put some cork into the clay which I have cut to look like rocks.
When the whole thing has dried and you have make sure the holes are sufficent to put your crew on it's time for some sand which I applied with PVA. When dried you do whatever you do with bases, I base the whole thing with Vermin Brown which I then highlight with a mix of Vermin Brown, Skull White and Tausept Ochre I then shade the whole thing with Devlan Mud. I then painted the rocks with Codex grey, shaded with Badab black and then Highlighted it with a mix of Astronomican Grey and Skull White. On these rocks I also had some moss which where some sand/PVA mix painted Snot Green and shaded with Thraka Green.
I then added some clump foliage, Middenland tufts, Mordheim tufts, static grass as well as Army Painters Brown Battleground. And Voilà done.
And here you can see the result with a Mortar and it's crew and everything. Hope you find this tutorial somewhat useful and if you have other and maybe better methods of doing similar bases please let me know.