the easiest way is to create a new object and start typing either "cv.jit.___" or "jit.___" and select any of the object expressions that appear in the drop-list. then (in edit mode) alt+click the object and learn more about, copy it, edit it-whatever you please.
most of these geometric shapes can be created with jit.expr and jit.gl.mesh.
for a more concrete example, recently i wanted to create a spiral shape, so i googled spiral equations, or something like that, and found this:
on down the page if found the spiral equation in needed
x=tcos(6t) y=tsin(6t) z=t
i plugged this into jit.expr using norm*5 in place of t (found by experimenting), and plugged the output into jit.gl.mesh. viola.
'jit.expr @expr (norm*5)*cos(6*(norm*5)) (norm*5)*sin(6*(norm*5)) (norm*5)'
hopefully this is helpful, but if not, as mentioned, stay tuned.