As a quick little guide, I think there's a key difference in how to go about describing either your main character (MC for later reference) in a novel among the three methods to write: first, second, and third. I myself probably don't give enough information on the physical features of characters, and instead, go far more into their body language and things related to that, but I'm trying to work on that.
Since the first point of view is meant to guide the reader directly through someone's mind using "I/me" as a reference, there can be some issues in describing either the narrator or other important people in the book. For the narrator themself, there are a few techniques to throw in specific to just the first person. One is using comparison to your benefit - compare the MC to another character, examples being a family member, a friend, or a celebrity. This way there can be easy descriptions - such as, "I must have borrowed my brown eyes from my mother, but definitely not her blond hair." Another method would be dropping slight hints into dialogue from other characters, going from a supportive friend that compliments your outfit, or the opposite, something snarky from someone you can't stand.
The second point of view is not widely used in most circumstances, but I would like to give some advice for this view either way since I think it can be quite effective if used well. Since the second person is meant to connect with the reader in a more unusual way, there can be some other methods on how to describe the main character that don't mimic a laundry list of traits. One could be the ever-classic looking in a mirror or a window, especially if that's something the MC likes to do, is watch their reflection or other people's. From there, give a more realistic thought process of going through what they see in a mirror, such as, "You look over your flat expression, wincing at the large bags beneath your eyes. Sleep hasn't come easy in a long time. None of what's happened recently has helped."
Finally, for the third point of view, this can either feel detached like reading a book with a narrator separate from the story, or really close and personal if the narrator is limited to only their own thoughts. There are quite a few ways of doing this, going from whether the view is omniscient or simply limited, but a few general ones are as followed. One good idea is to avoid dropping needless information. The reader is actively building an image of what the MC may appear, so I tend to vote for not giving too much of the regular info away and focus on various little things - scars, freckles, moles, any of that stuff gives a unique look into the person, and more so how they dress as well. Something like, "She tugged a baseball cap out of her pile of hats, one dedicated for every weekday," gives a neat insight rather than just, "Her brown hair is pulled tightly back to allow for loose strands to cup her face and highlight her pretty blue eyes."
Of course, this is my opinion (based on advice from other websites as well), but I think it's an interesting place to start a novel, with what approach someone will have towards their main character. Have fun writing. ^^
Possibly Useful Resources:
http://bekindrewrite.com/2012/06/22/6-w ... hemselves/
https://writersbeat.com/viewtopic.php?t=23581&=1
