Two ways:
promotion (the easy one, you won't see any graphical difference between your custom widget and the parent one, cause it "promises" you only the fact that setupUi method will create a widget of your custom type, and not the original one). In this case, just right-click on your widget, promote it, give your .h header file and the type of your custom widget.
writing a component for Qt designer (the long one). In this case, refer to Qt documentation, just look for "Creating Custom Widgets for Qt Designer" in Qt Assistant, or have a look to this example: "Custom Widget Plugin Example":http://doc.trolltech.com/4.6/designer-customwidgetplugin.html
Tony.