can anyone explain

I apologise first up for being so ignorant on the following question, however I can't quite understand the process that is occuring in the drilling at present.
Could someone please explain what actually is occuring with the drilling and how the BOP is secured.
I understand that cement is poured behind the casing,is this around the casing and at what level? ...it also has been mentioned that mud has to be removed or packed? Smaller casings toward the greater depth and Calypso.
Maybe a diagram could be thrown up?

Once again my apologies for asking the obvious but I am dead set interested and would appreciate a heads up on this.