Support the exterior window sill - how to do it properly?

Hello,

at two basement windows, additional masonry work needs to be done to install granite window sills. What is the correct procedure here? I would place 5 cm aerated concrete blocks into the mortar bed and then also set the granite window sill into the mortar bed with the appropriate slope outward. Or is that botched work?

I also wonder what goes between the mortar bed and the Styrodur? And does something need to be placed between the frame extension at the bottom and the aerated concrete blocks?

I am grateful for any tips.

Or is that botched?

Yes. Sealing layers? Plaster connections? How is all of that supposed to be done?

That's why I'm asking the experts here (see headline) ;)