The use of a picture shape allows a precise cut.
You outline the chosen shape on a picture, using a felt pen for watercolours, then cut out this shape, simply using scissors.
There are picture shapes in rigid plastic, allowing a direct cut.
The felt pen is not needed any more, you cut out pictures using a scalpel or a cutter.
A more precise solution, but much more expensive than the traditional picture shape.
A guillotine suits well for straight cuts and straight angles.