Why invisible mounts ? Just because it is very nice, and because I like to do things the complicated way...
Step 1: Plan the Cut
Note the cutting lines done by hand will be on the wall (understand hidden...), and the nice machine straight cut will be the one that everyone will see !
There are actually three shelves, two are done for the moment, the last is about to come.
Step 2: Walls and Corners
Then in most situations you will have your cut shelve, but unable to match it with the wall, as with A or B situation.
What I suggest is to take an angle, and a pencil. Put the pencil through a hole of the angle, and draw the profile of the wall on your shelve, as in the picture.
When you cut your shelve, you will have a perfect match with the wall.
Obviously, everything worked for me the very first time : The line was perfect, I hold the jigsaw 90 degrees to the board, and I did not need any sandpaper work...
Step 3: Mounts and Mounting
But then, some problems occur, on how are you going to put the shelve in place... The process that my father tough about is illustrated in the second picture. The first side of the triangle is pressed in the isolated anchor in the wall, and then the shelve is slided into the two anchors on the other side of the triangle. The picture is better than my explanation, actually...
This leads to a mandatory gap inside the thickness of the shelve, to allow the sliding process. This is why, I needed to cut an elongated hole on one side of the board., as the 4th picture show.
Step 4: Wall Preparation
The first picture show a little preparation of the plastic anchors : the winglets have been cut, because the wood pins could not entered the anchor, when installed in the wall.
Also, what I suggest is to drill a hole to allow insertion of the green anchor until 2 or 3 mm remains. Then, a hammer helps to make a perfect match between the head of the anchor, and the wall.
Step 5: The End ?
I still want to do some sandpaper work on the side of the shelve, to round it, so it has a better finish. Also, I want to do some dyeing, so the pine wood will look better.
I hope you appreciated this instructable. I took me time to do it, because I do not have any table saw, nor sharp drill. I did everything, holding level with my hands. As usual, any comments or suggestions are welcome...