Step one is to eye out the general line you want to follow, then give yourself a little extra length to even everything out once the first cut is done. Eric recommends picking up a pair of scissors from Beardbrand for the highest quality scissors you can get.
The biggest thing you want to remember is to follow your jawline. You want your beard to shape and contour with your jaw, and you want to continually brush the beard up and away from your face to locate loose ends and eliminate them.
Take your time! Don't rush your trim, in fact it may take two or more small trims over a few days to get your beard exactly where you want it. And if you make a mistake, don't fret - it will grow back!