ok so this is what i used to make my brothers lava base ( i agreed to completly make his first khorne brezerker for him). his base is made of this clay like stuff (not green stuff) and is very easy to make.
1. you just take a small lump (like on the edge of the block of clay) then mold it over the base you want it on.
2. now if your like me mind tend to turn into mini hill then just take ur modeling knife and slice a good amout of the top off to make it flatter (make sure you keep the sides the way they look when you first press the clay on that way it looks more natural
3. now your going to want to make sure your clay isnt to high if it is just do step 2 until its at a good hight (if it looks unatural on the top leave it once you make the depression for the lava flow it will look alot more natural). when you have it at the hight you want make to line in the clay with ur knife to show where the lava flow is, then proceed to carving, scraping out getting most of the clay out from inside the lines. this is the point where you will know if your clay is to high as before just to step 2 and trim the top until it looks right.
4. this would be a good time to let the clay harden. once you have the depression take a small amout of clay and roll it out into a really thing string and lay it inside where you lava is going to go. now take your knife and press down only with the tip on the string of clay to spread it out and hopefully if you do it right will give a flow-ish look to where you lava will go.
5. now just proceed to painting it (i will post a tutorial on this sometime in the next few days)