Don't overmix the batter. Stir just until most of the dry ingredients have been incorporated. Then add the cranberries and nuts. Finish mixing everything together with a few folds.
Use paper cupcake liners or lightly grease your muffin tin.
Muffins are done when they are golden, and a toothpick inserted into the center of a muffin comes out clean or with just a few crumbs. If there's batter, they need to cook longer.
Cool muffins before storing in an airtight container.