public class Contents 

internal class MyCustomPaneViewModel : ViewStatePane, IContentsProvider, IContentsControl {
    ...
    private Contents _contents;
    
    public Contents Contents  {
       get {
      if (_contents == null) {
        
        //Instantiate your UI (probably a custom user control)
        FrameworkElement contentsControl = new MyCustomPaneContentsUserControl();
        contentsControl.DataContext = this;//set the View model to the data context (standard MVVM)
 
        //Make the ArcGIS.Desktop.Core.Contents control which is used
        //by the contents dockpane to show your pane's content
        _contents = new Contents();
        _contents.ContentsView = contentsControl;//Your UI
        _contents.ContentsViewModel = this;//used for IContentsControl, IContentsPaneClipboardOps and INotifyPropertyChanged
        _contents.OperationManager = this.OperationManager;//Your operations manager
     }
     return _contents;
   }
}