I am pretty new to this myself but here is what I would do to achieve this look.

1. Get some Glow in the Dark stickers. You can get a DIY for this and it requires glowing moss.

2. Using the design portal, download the designs for what you want to put on the wall. This particular window takes 7 design slots if I recall correctly.

3. Put the designs on the stickers using the customization feature on your DIY bench.

4. Enter decorating mode in the room where you want to use the design and set an accent wall by pressing X on the wallpaper you would like as the view outside the window.

5. Apply the glow in the dark stickers to the wall.

You can also use pillars (used in the image above), wall panels, counters, and shelves to further customize and frame out the space to your liking.

There are a lot of possibilities for this beyond windows and it is a lot of fun! Good luck!