With great care..and depending on age.
A low vacuum and probably nothing else, trouble with (true) Silk Lampshades depending on their age, is they become very brittle with use over the years, sometimes its just the dust thats holding them together.If you apply a vacuum you could quickly see it fall to peices before your very eyes and disappear up the hose.,, same applies when using any dry clean solutions on them.
As usual test one first any indication of fatigue I would leave well alone.