How can I add the three objects to the main window?

[KaffeMitMilf]

Hey, could somebody please tell me how I can add the three sliders and labels to the main window, so if i am resizing the main window it does too?

[JonB]

@KaffeMitMilf
Before progressing your centralwidget is missing a layout. See the pane at top-right, where it shows red "no entry" sign on that widget name. You must fix that in anything you design.

Select your centralwidget, right-click, Layout sub-menu, assign a layout. Then see where you are. Layouts are required for widget resizing.

Layouts are explained in docs.