Firstly I grabbed three 60mm bases that I had lying around. For this style of base you ideally want the thin bases rather than the standard ones.
You'll also want some putty to build up the base, I use a product called Milliput which is relatively cheap and ideal for non detailed work like this. It costs $10 to $12 Australian and can be picked up from most hobby shops.
Lastly you'll want some pieces to add detail, since my army is based in a ruined city, I cut up a bunch of plasticard so that I had it all ready to go for when I needed it. You should do this before any other steps so that you can detail the bases whilst the milliput is still curing and soft. You can see I have a lot more than I need because I also cut up the stuff for the wrecked vehicle markers at the same time. I used a variety of pieces from rod, to I beams, corrugated iron and tiles.
I was almost ready to go, but I had one more step. To make things easier when building up the bases, I grabbed some spare 25mm round bases and blu-taked the underside of them. I then stuck them onto my 60mm bases so that I had the perfect reference for where my model's bases would go.
Now onto the fun stuff, I started by rolling out lengths of milliput and placing them roughly around the bases to define the edges. I then built up the milliput around the rest of the base into the rough shape of how I wanted it to turn out.
I then used my modelling tool to make a clean edge around the bases so that the infantry would fit in snugly but could still be easily removed for storage and representing wounds taken.
After that I added the plasticard and smoothed out the rest of the milliput.
Once it was dry, I painted on slightly watered down PVA glue, careful not to get any into where the infantry bases would go and then covered the entire thing in sand for that gravely texture.
They have since been painted but I haven't taken photos of those yet. I'll get some photos when I finish off the wrecked vehicle markers, hopefully in time for Cowcon this coming weekend.