It’s all done with mirrors…..well, a mirror.

If you photograph flat artwork and you don’t have the camera centred over the artwork and on a line perpendicular to the surface, you will discover the keystone* effect when you come to print the image.
*The square or rectangular artwork will be tapered in one or both axis.

To avoid this problem you need to align the artwork and camera precisely; some use a piece of string or a tape-measure to check the distance from both sides then from top and bottom of the art to the camera lens; a fiddly and inaccurate method.  Many art museums have an easel and camera mount built to precise angles and proportions to photograph art for cataloguing and insurance purposes.  We can get the same precision by optically aligning the camera to the artwork and it is simple and
fast.
Mount your artwork on a vertical easel or flat against a wall.  Find the centre by holding strings across diagonally, from corner to corner.  Place a flat mirror against the surface at the centre-point.
( to protect delicate paintings or art, a thin layer of soft foam or velvet can be glued to the back of the mirror )

Now mount your camera on a tripod or whatever fixture you plan to use, and move it till it is reflected in the mirror when you look through the viewfinder.  Make finer adjustments till the mirror is in the centre of the viewfinder and the reflected image of the camera lens is in the centre of the mirror.  The artwork and camera are now optically aligned. Copy your artwork with confidence.