Start by wrapping your box in white construction paper and cutting a hole with an X-acto knife or box cutter. (I used a Capri Sun box just to give you an idea of the size I used)
Cut out shapes from craft foam (I started with the face)
Use hot glue to put together the dog's face and use a permanent marker to make his mouth.
Glue legs to the side of the box and use permanent marker for details, then add more "spots" to the dogs body.
You can even add a little tail if you'd like
That's all there is to it. Now your child has an adorable dog valentine box to bring to school!
Notes
The amount of white construction paper you need to cover your dog will depend on how big a box you use. I used two pieces to cover the Capri Sun box and had some to spare.
Also, the time it takes to make this will vary based on the age & skill of the child (and also how much of a perfectionist your child is!)