The sun glory (circular rainbow) and Brocken spectre (shadow on clouds) are impossible to predict, but you need to be above clouds or fog to see them, like on a peak or canyon rim (or an airplane, but that’s cheating).
The sun needs to be low in the sky; aim to shoot near sunrise or sunset. Position yourself Just like with a normal rainbow, glories happen opposite the sun. Get the Brocken spectre by putting yourself in between the sun and the rainbow, so your shadow projects onto the clouds. If possible, move to a spot where you can also see some of the surrounding landscape; that’ll make the shot more interesting.
Expose for the highlights
It’s better to have parts of the scene too dark to than lose detail to blown-out highlights. Shoot RAW format if you can, and make sure the spectre is in focus.