oK go ahead and cut out the plywood piece. You should have now. A piece of plywood 3/4 of an inch bigger on the sides and the top than your wallpaper border and the width of your trim on the bottom.
Bottom rim is even and 3/4 on the sides and top. You can sand the edges of the plywood to get rid of any splinters.
Put some white glue under the trim and press down.. Hold it and flip the plywood and trim over so the finished side is up.
Put a few nails thru the finished side into the trim..Make sure the nails are shorter than the thickness of your trim so they don't go all the way thru the trim. Make sure the bottom edge stays even and the side and top still have a 3/4 space.
Flip your plywood back over and it should look like this now.. The trim is glued and nailed. You are now ready for the sides of the shelf. Get your new wood. I used 2....3/4 x 3 1/2 x 15 inch long piece as that is what I had. You need 2 pieces. One for each side of the shelf.
OK Here I lined up the top of my new board EVEN with the top of the wall paper. The new board is laying on it's edge and on the 3/4 inch space you left on the sides and top.
Make a piece of paper the same size as your new wood..side.....
Draw on the wood.Cut it out on your scroll saw.Sand the sides and bang the edges on the end of the belt sander.. You can also sand it by hand..
Now make another one exactly the same for the other side of the shelf!
Hand sand too if you don't have a belt sander.Now lay the sides on the plywood in the 3/4 spaces you left on the sides of the wall paper. Measure so the side board is EVEN......... with the top of the wallpaper NOT THE TOP OF THE PLYWOOD. You need a 3/4 space at the top.. Draw a straight line and cut the board. Now sand the edges at the top.. And get your glue ready.Put some glue on the ends of the trim.Put the side piece on the plywood on the 3/4 space you left on the sides of the wall paper. And even with the top of the wallpaper. I'm gonna post this so I don't loose it and finish the shelf in part 2.