Rectangle
create a new text element, delete the text in the right menu HTML section, go to the right menu Styles section add something like
to This Element
Circle
same as above but in addition add something like
the border radius should be at least 1/2 of the width/height of the element (see right menu Properties tab). For a perfect circle set width = height (insert the same values)
All other shapes:
create a shape, copy the CSS code
and past it into the style of This Element
This tutorial also works for images! However rectangular image clipping (cutting of the sides of an image) works a bit differently, and will be described in a different tutorial.