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How are interior window sills installed in drywall?

I need to installation interior window sill in drywall.

1.) How are they installed in drywall? With foam?

2.) Are there technical arguments why interior window sills are mostly installed one particular way, what I have seen as a layman. Probably comes from "normal" plastering, because only after the installation of the window sill is plastered.

3.) Or is drywall completely flexible and you do it to your preference? 

• drywall
• window sill