Here is my step by step production of a tree in Unity. It wasn’t too hard, more time consuming. Anna Beth explained that this was an easier way to do it, but for a more precise realistic version, this should be done in Maya and then imported into Unity, I agree.
You Start off with the main trunk, and then add branches above it in the menu, it is pretty self explanatory and easily done in the inspector.
When a descent amount of branches stem from the primary trunk, you then add branches to to the secondary ones.
On the correct outer branches we then add foliage by pressing the add leaves button, moving the slider depending on how much bush you like.
By this stage it was looking good but more leaves were needed.
Et Viola…a pretty good tree for a first attempt. Could do with a bit more bush, but generally quite pleasing to the eye.