Just treat the canvas like any other object you place on the grid and set it to span the entire grid
<Canvas Grid.ColumnSpan="999" Grid.RowSpan="999">
<Button Canvas.Left="200" Canvas.Top="100" Width="100" x:Name="btnMyButton" Click="btnMyButton_Click" Background="#77ffffff" BorderThickness="1" BorderBrush="#77000000">My Button</Button>
</Canvas>Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.