So you want to know how to do lava bases? Well, here is how I do them, step by step with pictures so you can easily follow and have a reference for what each step should look like.
Enjoy!
1. Create the "lava" by using GreenStuff to cover the entire base. Press a couple big rocks into it, and cover the tops of the rocks with some sand for extra texture. You can add some lava bubbles by making a small ball with greenstuff and adding it to the base. On this base, I made a couple bubbles and added a popped bubble.
2. Prime white. White it easier to paint over than black, but if you only have black that works too.
3. Cover all the lava in Mecharite Red. If you get some onto the rocks or black part of the base--no worries. We'll clean this up later.
4. Cover all the Mecharite red with Blood Red.
Steps 5-9 are a Condensed version of the steps. Basically, you add a color, wanting the lightest parts on the edges and the "shallow" parts of the lava. As you add each new color, go as close to the previous one as possible without covering it up. It also helps to water down the paints and do multiple layers of the same shade. Basically, the more time you want to spend on this, the better it will look. If you want to add more gradual mixes, go for it. The more layers you have the better it will look.
5. Use Blazing Orange to cover MOST BUT NOT ALL of the red. Leave the deepest parts red, but you want the majority of the base to be Orange.
6. Use a 1:1 mix of Blazing Orange and Golden Yellow, repeating the same idea: cover all but a sliver of the previous color.
7. Repeat again, using pure Golden Yellow.
8. Repeat yet again, using a 1:1 mix of Golden Yellow and Skull White.
9. Finish it off by adding pure Skull White to the tips of the lava bubbles and the edges of the lava. You can also add some cracks to give it some texture.
10. Cover the rocks and the edges of the base in Chaos Black.
11. Drubrush the rocks with some Codex Grey. I did this is three layers: 1:1 mix of Grey and black, a layer of pure Grey, and finally a layer of a 1:1 mix of Grey and white.
12. SOURCE LIGHTING! If you have some lava bubbles, add some source lighting by drybrushing some Golden Yellow with a touch of Orange mixed in onto the areas near the bubble.
13. Repeat, but this time with a 1:1 mix of Yellow and White, concentrating on the areas closest to the lava and closest to the bubble.
14. ENJOY! Here are some pics of my finished bases:
I hope this helped, and that you enjoyed it. Let me know what you think!
Thanks and good luck hobbying!