The way I did it on my "Leftovers " miniature in my WIP was to put poyfilla where I wanted the lava to be and then pick at it with a cocktail stick to make the surface uneven. Once dry paint white then drybrush heavily with golden yellow followed by lighter drybrush with blazing orange followed by lighter again drybush with blood red. Last drybrush with scorched brown. Once dry ink with yellow ink. last of all light drybrush with chaos black.
Hope this helps you matey
: Edited for spelling mistakes and colour names
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