m_) good luck making the set.

Making leaves out of clay might be hard, so you could try and use tissue paper for the various foliage.

Edit: Oops, I didn't read the part about you just clumping the clay and putting it on the trunk of the tree. Oh well, I'll leave the suggestion for anyone who likes the idea

I like the ones you made.

I usually use some wire or a bake able clay like sculpey and make a tree shape out of it.
I then cover it in Van Aken or whatever clay you use, and then just add some texture with a tooth pick.
You can add on green to make it look like leaves.
Hope that helps a little.