I'm finding it hard to do a tutorial when I have no idea what I'm talking about.
So I guess I will just wing it ..ha!
I used one board for my base, mirror from Wal mart,decorative wood garland from Home Depot and trim from my sisters garage :)

I set my pieces where I wanted them to go .... measured by sight then cut.
You can see here where I measured by drawing lines.
I didnt measure for perfection...just looks.
I used this good ole handy dandy saw to cut my edges.
Before you decide on a mirror size I would measure your wall space.
Cut out a piece of paper and hang it on the wall for a few days.
Move it around..
I have high ceilings so the long mirror worked perfect for me.
I also found that drawing lines helped me keep things in their place.
Make sure you paint your base before you add the mirror.
Opps. I forgot to tell ya'll I attached my trim with small nails and wood glue.
I told ya'll I stink at giving tutorials..sorry..
Dont forget to paint your mirror trim before you glue it down.
I used liquid nails to hold my mirror in place.
Once my mirror was glued down I then attached my trim .
I sanded and stained..and sanded..and stained...
till I got it just right...
see...
I must read a tutorial on how to write a good tutorial..ha!
I'm one of those that builds like I cook.
No recipe..
No correct measurments...
No rules..
Just right~