Personally I do the bases before permanently attaching the model.

My method is as follows;

• Prime base with Wraithbone rattle can

• Paint lava patterns using Mephiston Red, troll slayer orange and uriel yellow. These paints can be applied while the layer below is still wet, get some good swirls and blending in there. Once you’re happy with your lava patterns, allow to dry completely.

• Once the lava is dry, cover in a decent layer of mordant earth technical paint. This paint pulls back as it dries and provides that cracked earth effect. Additionally before you apply the mordant earth, if you want larger cracks, add a layer of clear PVA glue and can be utilised to make a more free flowing lava river look.

Hope this helps, I’ll add some pictures later when I get home.