It's not easy to trim a goatee. It took me a while to get it right and I had to get help from a friend for the first couple of times.
The hardest part is making all the hairs the same length. The tips mentioned in the article are good. I use a small beard comb and cut the tips with a small scissors. I use an electric trimmer to manage the shape in between trimmings.
Getting the angle right can be difficult and takes some practice. I like to do some simple measuring with my comb on both sides of my face to make sure that everything is even.
Applying a few drops of beard oil also makes my goatee look nicer in between trimmings.