While you will get a better result with a blender, if you don't have one, you can still make these milkshakes. Melt the Nutella with a splash of the milk and mix until smooth and thinned out, then leave to cool slightly. Soften the ice cream, then mix it with a small amount of the milk with a fork or whisk, then add in the rest of the milk and the Nutella, and mix. Your milkshake won't be quite as creamy and smooth as one made in the blender, but it will still be delicious.