Doing the basing shown above isn't very hard, but it does take a lot of time and a lot of steps.  The steps are these.

1. put the figures on the bases with something temporary -- blue-tac, two-sided tape, whatever.

2. smooge white glue around the base with your finger or a crude brush, trying to get everywhere that isn't figure.

3. immediately take the figures off the base and shake coarse sand on the base.  The size of the grains of sand depend upon the scale of the figures; you can sift the sand in advance to separate out the larger pieces from the smaller.

4. Let the glue dry.

5. Spraypaint the base white or black.  If white, make sure you get all the central area near the figures.  If black, make sure you really get everything away from the figures.

6. If you sprayed black, when it is dry you need to slop white on the central area near the figures.  Maybe two coats.  If you sprayed white, you need to slop black heavily in the areas that won't be "hot" -- not near the figures.  Might need two coats; make sure you get the nooks and crannies.

7. When dry, take four paint colors: bright yellow, orange, bright red, dark red.

8. Using the paint colors in sequence, paint four rings from the outside in, starting with the dark red and ending with the bright yellow.  Since you're using yellows and reds, which are usually quite translucent colors in craft paints, you might have to do two coats of each ring.

9. Using the paint colors in the reverse sequence, touch up the exposed surfaces (which would be exposed to more heat/light).  Use yellow in the orange ring, orange in the bright red ring, bright red in the dark red ring, and dark red on exposed surfaces in the part of the black area that is near the red ring.

10. Put the figure down on the base and check your work.  Notice where dark colors are too close to the hot figure, or light colors too far, and touch them up.

11. Keep doing 8 and 9 and 10 until you can't stand it any more, then declare your work "good enough" and quit.

12. Drybrush the black rocks with a bit of white.  Which will mean you might have to touch up again with step 11.  Curse a little, but do it.