Good advice so far just thought I would throw in my two cents also I am a mixed shaver as for work I have to be clean shaving but it doesn't take me long to grow in a goatee so if I get three or four days off I will grow it in so don't get as much practice trimming as others but I found that three strokes work real well for me when I am trimming. First things first shave normally going as close as you are comfortable going to the goatee and then leave be the rest till you have shaved the rest of your face. The first stroke is the vertical stroke already mentioned I will do this both from goatee towards the ear and ear towards the goatee. When going ear towards the goatee go real slow and as you reach the edges of the goatee you can take individual hairs off just go slow. The next two strokes I use you may have read about using on the neck which are an downward scathe and an upward scathe. These are where you are moving down or up and across your face and need to be VERY SLOW and VERY LIGHT! But what I do is for the top edges I start at about the top of the cheek bone and come in to the top edge of the goatee and for the bottom I start at the jaw bone or under the chin and come into the bottom edge. Again with these two strokes they must be VERY SLOW AND VERY LIGHT or you will cut yourself very badly.
Hope this helps some.