I have tried quite a few different products to fill them in but I always go back to the Sleek palette in Storm, I use the medium brown on the bottom row, second last.
I like to use my Mac #266 angled brush as it gives good control and I just love it.
I then either use a plain mascara wand just to shape them or I will use clear mascara like MUA Clear Mascara which is only £1 and does the job, only problem is that it is a clear tube and it does get a bit disgusting!
I then use my YSL Touche Eclat under my eye brows for a highlight. I like it as it is natural but looks amazing.
Its very easy all you need to do it use a light hand because it is easier to add more product than try to take away, and follow the natural shape of your brows.