Summarize:

Now you want to make sure that fade has a distinct ending point at the base of the head and bottoms of the sideburns. Take your trimmer or clippers and shave any remaining hair away. You may not need to do this to the sideburns if the person whose hair you are cutting has facial hair connecting to the sideburns. Shave down the base of the neck and up a little way on the lowest fade to make sure that the hair is evened out. Look for any patches where the hair is uneven now and make sure to go over them again to clean them up. You may have to position your clippers at a bit of an angle to get any stragglers and even out the fade. Now that your bottom fade line is established, cut the hair beneath it. Make sure you get the sections at the bottom of the neck that have thinner longer hairs. Clean up the edges using a straight razor or the trimmer to make sure the edges of the cut are clean, as well as removing any neck hair. If you are using a straight razor, apply shaving cream to the area you are about to cut, then pad with a warm damp towel. Brush off any loose hairs and style the top with a pomade or gel for a finished look.
Shave the bottom of the hair with a beard trimmer or unguarded clippers. Blur out any remnants of the visible line using your clippers. Clean up the base of the neck.