To use a green screen in CapCut, first, you need to have green screen footage and background footage. Open CapCut, import the green screen footage, and drag it to the timeline. Then, import the background footage and place it on the layer below the green screen footage. Tap on the green screen footage, select the “Chroma Key” option, and choose the green color from the footage using the eyedropper tool. Adjust the settings as needed, and you should see the background footage replacing the green screen footage. Finally, export your edited video.