You can check the video or written instructions below:
Create a sloped attic ceiling with an opening
Instead of using one large object, you'll create the ceiling from several separate pieces.
Prepare your attic room
Open your attic project.
Select the room and hide its ceiling if it blocks the space you want to create.
If necessary, hide any existing walls that interfere with your intended roof slope.
Switch to 3D mode to check the available space.
Create the sloped ceiling
Add a Magic Cube to your project.
Resize it into a large, thin rectangular panel.
Switch to 3D mode and raise the panel using Levitation.
Tilt the panel to match your roof slope. Select the panel and enable 3-Axis Rotation in the bottom toolbar. If the option isn't visible, go to Settings and enable 3-Axis Object Rotation.
Adjust Angle X, Y, or Z, or drag the colored rotation rings, until the panel follows the desired slope. Change one axis at a time to avoid rotating it in the wrong direction. For detailed instructions, see How to Use 3-Axis Rotation.Position it so its lower edge meets the low attic wall and its upper edge follows the roof slope.
Adjust the dimensions, elevation, and rotation until the panel fits your attic.
If you need to create a custom low wall, you can use another Magic Cube, resize it to the required height, and position it underneath the sloped panel.
Create an opening for a skylight
To create an opening in your ceiling, you'll need to use multiple ceiling panels.
Duplicate the sloped ceiling panel.
Resize and position the pieces around the area where you want the opening.
Leave an empty rectangular space between the panels.
Make sure all the ceiling pieces follow the same slope.
Apply your preferred ceiling material.
Switch to 3D mode and check the result from inside the room.
Important: This technique creates a visual opening in the ceiling. If you want an actual skylight with glass and a frame, you'll need to add those elements separately. Also make sure no existing roof or ceiling covers the opening.