Paint.NET: How to Blur Faces

How to blur faces (or any part of an image) in the Paint.NET photo editor.

Open the image.
Choose the appropriate selection tool from the upper left of the toolbox.
Select the part of the image you want to blur.
Go to Effects > Blurs > Unfocus.
Move the slider to select the amount of blur.
You can also use Pixelate under the Distort menu.
Deselect the selection using the blue box and red X button in the toolbar.
After performing one blur, there is an option at the top of the Effects menu to repeat the blur.