STEP ONE: SO. Firstly, I roughly fill in the main part of my brow using Bare Minerals' Frame & Define Brow Stylerpencil in 'light'. It's a pretty perfect match for my natural brow colour, so works well to make them look a little fuller than they really are. If you're more into a natural looking brow, you could get away with stopping at this step (and maybe adding a little brow gel for good measure).
STEP TWO: my absolute absolute absolute brow hero product of the moment is Anastasia Beverly Hills' DipBrow pomade in 'auburn'. This is hardly revolutionary - every beauty blogger in the entire universe has raved about Anastasia brow products at some point in the past few years - nevertheless this is what I use every day. If I was only allowed one brow product, or if I was packing a small make-up bag of essentials only, this would be that one. I apply it using an Ecotools angled brush - any angled brush will do, but my personal preference is for a very thin one that allows me to be more precise.
STEP THREE & FOUR: as well as DipBrow, the very essential step in my eyebrow routine is to outline them using concealer or foundation on a small brush, to neaten them up. I use the Real Techniques detailer brush, from their Core Collection (if you don't want to buy a full set, although I would recommend the Core Collection, try this brush). I blend the concealer or foundation out into the rest of my make-up so I'm not left with giant white lines around my brows, and then finish them off with a bit of Eyeko's brow gel (which, after trying a few over the past few months, is my favourite) to help keep everything in place.