Take a scrap piece of wood. Draw a triangle shape with a straight edge.
Cute your triangle piece out with a Bandsaw, Jigsaw or Circular saw.
Using the straight edge, draw diagonal lines across the triangle.
Cut along the lines. You can use a Miter saw if your triangle isn't too wide.
It's more important that the inside lines of the tree are straight.
Paint and decorate however you want! I wrapped my pieces in craft wire and then glued buttons on with E 6000. I tried to glue the pieces of the tree to my board, but they wouldn't lay straight with the craft wire, so I shot them a couple of times with a pneumatic nail gun.
You can use a painted board, a cupboard door or a piece of bead board for the back.
I used a a little wooden rectangle that I picked up at Hobby Lobby for the trunk.
Which reminds me, I saw a sign for a new HL in American Fork! Yay for us that live close by!!!
Try to ignore the sad little drawer pull on the shelf.
I don't ever think about fixing it, until I'm going through pics!