Open CapCut and create a new project
Import the video you want to edit
Tap the video clip on the timeline
Duplicate the video layer
Place the duplicate layer below the original
Select the bottom layer
Apply a blur effect to the bottom layer
Increase the blur strength as needed
Scale the bottom layer up so it fills the screen
Keep the top layer at normal size
Adjust the top layer position so it stays centered
Trim both layers to match the same duration
Export the video when finished