There are many ways to skin this cat, Freehand, Jig, Straight Edge. I used 4 Straight Pieces of wood set so that when routing only the bit that I wanted to remove could be removed. The smaller the base of your Router or the bigger the work area the easier this method is.
Ideally use a plunge router and make sure that the height adjustment is set nice and tight to the correct thickness before you start
If you plan on doing this multiple times (which for me is more then twice) then for perfect results you should make yourself a Jig and use a bit with a top bearing.