1. You can't!
Well, not really simple BUT it's possible like in this picture:
i can find the velux object but I cannot get to change the angle to make if fit the roof angle
Export the window as object;
Import it as furniture( NOT as doors&windows);
Elevate it at 1m than rotate it how you like;
IMPORTANT!
Don't expect to act as a window in the wall.
Will don't cut the necessarily hole in your roof.
At this moment it can't even on my roof.