As I'm typing this I have no idea how I look. For all I know I might have a funny little smile on my face, or my brow my be furrowed, or my jaw clenched. I might even look as radiant as the morning sun. (I wish.)
Similarly, when we're looking at the world through one character's POV we have to remember that while they can see other people's expressions they can't see their own.
So you can have this:
Joe didn't know what to say or do. Helen looked furious. Her cheeks flared red as she spat out the words, 'I hate you.' He felt his jaw tighten in response as he stopped himself shouting back.
But you can't have this:
Joe didn't know what to say or do. Helen looked furious. Her cheeks flared red as she spat out the words, 'I hate you.' A muscle flickered along his jawline as he stopped himself shouting back.
So, how do you describe the viewpoint character? The answer is obliquely. (Not looking a mirror. That's a cliche.)
She sucked in her tummy and tugged as hard as she could, but there was no way the zip was going up to the top.
ie the character is plump
I ran her hands through my hair, wondering what it would be like to have hair as long and straight and blonde as Gwyneth Paltrow's.
ie the character has short, dark, curly hair
She heaved herself off the chair.
ie the character is large and ungainly
I hurdled the 5 barred gate easily.
ie the character is athletic and probably tall
I squirted extra sunscreen on my head and rubbed it in.
ie the character is bald.
It's not difficult to do once you get the hang of it. The other thing to remember is that you don't have to describe your characters in minute detail. If anything, less is more. You want to convey just enough information so the reader can create an image in their minds.