Donna,

You would get a much better job by making a scarf joint rather than a butt
joint. When the wood shrinks, and it will, you'll be left with a gap at the
butt joint and will be able to see through it; if you scarf joint it (make a
miter joint between 2 pieces, the gap will have cedar behind it rather than
air.  And yes, the scarf joint is harder to make, but with a miter box or
chop saw, you can do it.  Practice on scrap before wasting nice length of the
good stuff.  You also should lay out the corner joints (which should NOT be
simple miters, because when the pieces shrink they joint will open up; you'll
also find out by mitering that the corners of your room AREN'T SQUARE. They
never are.

Put up a full length piece on the wall opposite the entrance to the room (or
the most prominent wall), and then do the pieces on the 2 walls abutting;
miter the corners on the prominent wall, cope along the miter, and cut the
back ends square to fit flush against the 4th wall.  Last, do the least
prominent wall, with both ends of this piece mitered and coped.

It's a pain in the ass, but it's the right way.  Not that I've described it
well, nor can I give you a book reference, but there must be practical
carpentry books (or Fine Woodworking mag) that will explain it better and
with illustrations.

Good luck.

Ralph