There is no one-size-fits-all answer to this question, as the best way to fill your eyebrows depends on your own individual features and brow shape. However, some tips on how to fill your eyebrows naturally include using an eyebrow brush to brush the hairs inwards towards the center of your brow, using a light brown eyeshadow to fill in any sparse areas, and topping off with a light brown or black eyebrow pencil.