Step 3: Preparing the base and mounting the tree on it

You don't need to cut your board if you like the size.  If it is too big, measure a template out on a big piece of paper; like an old newspaper.  Cut this as straight as you can on a table saw.
 Drill a hole on one side with a drill, a bit off centered so when you put the wire end of the trunk through the hole it will be towards the middle of that side.
Sand it down making sure there are no splintered edges and clean it off. 
Now it's time to water proof so it doesn't rot from being wet from paper mache, or the plaster cloth.  Take your base coat and follow directions on the can, take the time to wait for it to dry because you want to do it right and not quickly.   You run the chance of messing up and having to redo it if you don't take the required time.  Now when that is done drying use the waterproofing paint and spay it down as even as you can.  Do the same for this one, and let it dry thoroughly.

Put the wire end of the tree through the hole and bend it down on the bottom.  Then position your tree on the top.