To start I brush through the brows with a spoolie - mine happens to be a Mac one as it came in a set, but literally any one would do - to get the hairs in the right direction, up and out at the front and middle, and then down towards the end. I then go in with a Mac eyeshadow in Charcoal Brown on a small angled brush, and draw them in. I fill in the natural shape of my brow to make them more prominent and to fill in any sparse areas. Then, right at the front of my brows I take the powder higher up, to make them look fuller. If I didn't do this, they would look a little triangle-y. Obviously, this isn't a step that everyone would need to take.
After this I brush through with the spoolie again, to soften the powder. Then I run Benefit's Gimme Brow Gel in Light/Medium over the brows. This sets them in place and adds a tiny bit more colour. I didn't think I would need this step, but I can honestly say that it makes a world of difference in making my brows look more natural.
Usually that is me done, but I included the Collection Lasting Perfection Concealer in the photos above for a reason. Sometimes - we've all been there - you colour outside of the lines, so to speak, and need to tidy up. I've found it's really handy to take a little amount of concealer on another angled brush, and to run it around the edges of the brows. This erases any wonky lines and makes the brows look much more precise. I don't do this every day, just if I go a bit overboard when filling them in!
I personally don't like my brows having a big arch, and so mine are fairly straight with a slight arch, and they are fairly thick too. This routine could work for anyone, however, so I hope this was useful!