Since you already have an undercut, the demarcation line should be easy to find. If you need to, use a mirror behind you to make sure that the line is completely straight. If you do accidentally catch your longer hairs in the clippers, you may have to make your undercut higher.
You could also put a thick towel around your neck to catch the hairs.
You can even use a 3 way mirror. Practice by starting the separation lower than you want it. So that you have room for error. If you’re nervous about trimming the back of your head, get a friend to help you out.
If you go for a longer guard, you’ll have to trim your undercut more often.
If you’re doing an undercut all the way around your head, like a men’s style, you can start at your sideburns instead.
If it’s your first time trimming your undercut, it’s better to go slow. That way, you can correct any mistakes if they happen.
Double-check that everything looks even in the mirror behind you.
Trimming the demarcation line is optional, but it can help make your undercut look cleaner and more professional.
If you want your fade to be a little shorter, use a #1. 5 guard instead.
If you used a #1. 5 guard all over, use a #1 guard for your fade.
Try to get your line mostly even, but don’t worry if it’s slightly wobbly. The fade will help to disguise any imperfections.
Make sure you use a mirror for the back of your head!
