There are so many products for your brows ranging from pencils, powders, gels and pomades, but the most versatile and easiest to use would have to be brow pencils. They don't require any additional tools to apply, and can be used to define the shape, as well as fill in sparse areas.
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SHAPE UP
Using a brow pencil, you can extend the ends of the brow, especially if they're lacking in fullness. Draw the ends and fill them in to create a seamless look from start to end.
FILL IN THE BLANKS
If the front of the brows are looking a little sparse, you can fill it in using hairlike strokes with a brow pencil. To help the drawn-in parts look more uniform with the rest of your brows, use the spoolie to brush it through to blend in the pigments.