XamlBindToViewModel.xaml

<Window x:Class="ProjectName.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:viewmodel="clr-namespace:ProjectName.ViewModels"
        Title="MainWindow" Height="350" Width="525">
  <Window.Resources>

    <!-- This creates an instance of the TestViewModel held in the ProjectName.ViewModels namespace -->
    <viewmodel:TestViewModel x:Key="TestViewModel" />
  </Window.Resources>
  <Grid x:Name="LayoutRoot" DataContext="{StaticResource TestViewModel}">
      <TextBlock Text="{Binding TestProp}" x:Name="txtBlock" />
  </Grid>
</Window>

XamlBindToViewModel.xaml.cs

using ProjectName.ViewModels;

namespace ProjectName
{
  public partial class MainWindow : Window
  {
    /// <summary>
    /// This property is if the ViewModel needs to be accessed in this codebehind
    /// </summary>
    private TestViewModel ViewModel
    {
      get
      {
        return (this.LayoutRoot.DataContext as TestViewModel);
      }
    }
    
    public MainWindow()
    {
      InitializeComponent();
    }
  }
}