Any roofers able to advise, please?

[Freeranger]

I know this sounds mad and it is, but needs must when the budget is tight. I have an (opening) window to fit as a rooflight, and I have a roll of lead flashing. I don't have pre-made flashing kits and it's a very old non-standard restoration anyway but without the cast-in apron, so it looks just like a window and frame. I also have a Scottish roof, so slates are nailed directly onto sarking boards. No felt, no battens.
My questions are: 1) do I recess the window (depth c. 50mm) into the sarking boards or do I place it on top of them? 2) what is the best arrangement for flashings?

I was going to make an 'apron' from a continuous strip along each side, working from bottom upwards, then soakers up the sides and a cowl at the top. I'm not sure how to make the apron waterproof at the join with the frame. Caulk? And I'm assuming I should bend the apron piece at 90 degrees to the frame, but I could make a flat piece, put the frame on top and seal the joins.....