Depending on the size of the cone, and the length you want the "branches" to be, you would use a star tip. I would not go any higher than a 16.
You might want to add a little more powdered sugar to your RI. Sift it first, to prevent lumps.
Theresa 
Help please. What tip do I use to do the pull out leaves in royal icing on a sugar come? I am trying to make a tree now and it looks awful. My royal is really sticky and keeps pulling out way too much
Sue
Have you had a look at this tutorial?
http://www.cakecentral.com/article55-Ice-Cream-Cone-Trees-Great-for-Christmas-Cakes.html
I found it really helpful, i made some trees using these instructions last year.
cheers
Nati
thanks so much everyone. I can't seem to find these tutorial when I look 
I don't have that tip Off to AC Moore to get one and hope they are open on X-mas eve! I tried a tree with the leaf tip # 67 but I am not happy with how it looks. If I have time I will try another one with the proper tip later. I am new to this and somehow got myself into 3 cakes for tomorrow 
What was I thinking.
You guys are the best, thanks for your help 
SW
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