STEP 2: Basic shader structure

A simple elongated sphere	Transparency added to the bottom half, with a smooth transition	Color gradient added

Doesn't look much like a flame yet. Changing the exact color later wil be easy, but first there are two major issues to work out. One, the bottom half of the sphere is in shadow. A flame has no shadows since it emits light instead of reflecting it. Two, the edges of the sphere are far too sharp and the transparent globe at the heart of the flame is missing.