The usual cover process begins with a very basic brief
from the editor.
-- Young Cyclops' eyes blasting uncontrollably.
-- Black Panther on throne, flanked by supportive Storm.
-- Black Panther fighting Killmonger.
-- Wolverine and Shadowcat leading the Knights of
Wundagore into battle.

More often than not the covers are done ahead of
interior art or even plots
(to allow the cover to be
promoted in catalogues.)
 Often so far ahead of interior
art that details are sketchy (which has lead to the
increase in generic, 'iconic', non-story specific covers).
In the case of the Black Panther versus Killmonger cover
there was no ref for Killmonger's armour so I designed
that and rationalised three different BP armour designs.

Its normal to send the editor two or three roughs to select

With the drive towards 'painted' covers I've recently
added crude airbush rendering to the roughs in the hope
of having the colourist model and light the image