How to make a cartoony tree

That's all done in a 3D modeling application and 2D image application.

Looks like you want to create several cones that are UV mapped as a circle. Create a texture with a cartoon-like leaf texture radiating from the center in a circle shape, with transparent edges.

The cones should be on top of each other and somewhat sloppily placed, so they look random and not perfect (this gives a better cartoon-like effect).

Create several different variations of the texture and tree, and import them into Unity. For an extra cartoony feel, be sure to use a good shader. You probably won't want to use the toon shader that is included in Unity as it likely would not give you the result you want. I'd suggest simply lighting the scene so it is bright and friendly, and tweaking the textures so they look good.

I hope this helps.