Honestly I have no idea what I'm doing now, I'm just doing what feels right. So first I glued a load of brass rods into the wooden base so that the clay has something to grip onto rather than just sitting on top of the base. I then started putting on the putty within the outline on the first pic. I had originally used too much so I used some tooth floss to remove the excess. Once I had smoothed the top with one of my clay modelling tool things I roughed up the sides of the tree trunk to make it look more like bark and then created the rings on the top.
I've no idea how long it takes for this stuff to dry so I'll just leave it for a day or two and then start on the cracked earth.