@CoolBeanz I'd completely disregard @TheShadow'sQueen 's advice if I were you.
I personally am not non-binary, but I know people who are, and used to identify as non-binary myself while i was still figuring myself out.
From what I gather, being non-binary is not a base gender, with extra fluff added on top of it. It is a completely separate thing from the binary genders; male and female, and making a non-binary character a "non-binary person with a base gender" kinda defeats the whole purpose of the label non-binary.
You can give them a base "gender" for their /gender expression/(example: non-binary character wears dresses, which are generally coded female), which is completely unrelated to gender identity.
The reason non-binary people wish to differentiate themselves from binary folks is for mainly just one reason: gender dysphoria.
As a trans person, I can talk about gender dysphoria with certainty in the knowledge i have, so take notes.
Gender dysphoria is being uncomfortable with one's gender as assigned at birth. It is the reason trans people transition from one gender to the other. The same can be said for non-binary people. They don't feel comfortable identifying as either gender, so they have their own, which is neither.
To assign a NB character a binary gender and then covering it up by calling your character they/them does not make them non-binary.
It does not matter how much a girl will say "im non-binary uwu" and says their pronouns are "they/them", if they are comfortable existing as a female, then she's probably not non-binary.
(Note: don't use this advice to call people out on their "BS" in real life. Different people have different circumstances where they might not be able to transition completely, or come out. Especially non-binary people who are so often seen as not a real thing. Please take people at their word in real life.)
It is not convenient to be something other than cis (for the love of god i hope you know what these terms mean if you're gonna write a non-binary character). Non-cis peeps such as myself face a bunch of obstacles, such as discrimination, exclusion, ostracization, and all in all just not a good time. THESE THINGS WILL AFFECT PEOPLE. Ask any trans or NB person.
(a note: some NB people identify as trans, some do not. For the sake of this advice thingy-ma-doodle, I will be referring to trans people as binary gender trans peeps, and NBs will just be NBs)
So you will definitely have to do your research over how coming out and/or transition affects people. And you'll have to do research as to what being non-binary even means (i can tell you with almost complete certainty you'll get different answers from different people). And a whole bunch of issues.
Also you'll have to do some world-building around non-binary people. What is your setting? Are non-binary people a normal and accepted thing? Are they almost completely invisible? Is a non-binary gender something most people don't even know exist? How does your character respond to that?
PLEASE ALSO REMEMBER: There is no cookie-cutter formula for how to write a NB character. Every non-binary person is different, just like every boy is different, every girl is different, etc. Your character's gender should not define them. Your character's status as a minority should NOT DEFINE THEM. I see this done wayyy too often where a character is just there for the sjw brownie points. Please don't fall into that trap. Make sure your character matters besides them being non-binary. If they don't matter to the plot, or feed negative stereotypes, cut them out of the story.
And now to you as a writer. You are not non-binary. This does not mean, however, that you can't write a non-binary character. Please by all means write minority characters. God knows we could do with some representation. But please just be careful of /how/ you represent minorities. That's really the biggest pointer I can give: Bad representation is worse than No representation.
However, you are not non-binary. This means you should not write stories ABOUT being non-binary. Let real non-binary people tell those stories. They need to be told correctly.
In summary: non-binary does not equal binary with sparkles, setting is important, do some research, and just don't feed negative stereotypes.
Definitely try to get the opinion of a NB person. They can probably tell you more than me.