Hello.
I´d recommend to go
→ Developer Tools → Extension Wizard
This helpful tool will put up a development system within slicer and comes with a working templet of a simple extension with it´s own GUI.
You can edit the python code in a text editor of your coice and play around with the GUI after editing it in the in the QT designer.
Reload the extension from within slider and directly test your changes, All basic and necessary functions for a slicer extension are included. But you will need to write some lines of code …
Best regards
Rudolf