The GUI script uses [Qt for Python](https://wiki.qt.io/Qt_for_Python) ([PySide2](https://pypi.org/project/PySide2/)). So you can make sure you have that package installed by running:
```bash
pip3 install --user PySide2
```
After that you can just run the script like so:
```bash
python3 ./RetroUFO_GUI.py
```
You can then just click the `Grab Cores` button at the bottom and then you should be all set.
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If you would like to grab cores for a different platform or architecture you can override which supported cores it grabs.
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If you have your core directory set somewhere special you can override where the cores extract to.