I'm looking to install some wall-mounted shelves onto my walls such as these Ikea ones: Ikea Lack Shelves
However, my apartment is very old and the walls (solid walls) are not perfectly flat; like if you places a long straight ruler on it horizontally, you will see that there are some gaps between the wall and ruler.
I also would like some shelves that are placed in the corner, but the wall corners are not 90degree right-angled, and is about 2-3degrees wider.

What kind of work/method can i do to make my walls more flat or corners right-angled?