How do you create a custom widget in Qt Designer?

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QT-Creator

Okay, so this is driving me CRAZY! I know how to implement custom widgets in Qt via the promotion method, but now I want to actually be able to view my custom widgets in the designer before building my application, and every tutorial I've seen online either uses the promotion method or is very poorly explained. I looked at the Qt documentation and I couldn't seem to get their methods to work. I come from a .NET background, so I was kinda hoping I could use something similar to MS UserControls. Only this seems much more complicated. Surely it can't be that hard... could someone please offer a better explanation? Thank you!