Ooh good question! I do write a few characters with npd as a past time, and I have it myself, so I can give some little tips here and there!
1. A great place to start is considering how it might have developed! Was the character brought up in an environment where they didn’t receive a lot of attention, or did they come from a background where their parents and the people around the constantly praised them? This puts you in a good mentality for writing a character’s feelings and emotions, as well as their motives.
2. Consider any opposing challenges or difficulties the character may face. Are they leaving their immediate support/family group and meeting new people outside of that circle, for example?
3. How well does the character managed their symptoms? Or in what ways do they do so? There is the usual take of “character elevates their self-value”, but there’s also other varieties such as pwnpd becoming people pleasers (in order not to seem full of themselves; especially if other characters have accused them in the past.. depends how in depth you’re going for this character in terms of writing their backstory). Depends also on the time period and setting of your character - if it’s a modern time, they may possibly have a diagnosis for npd and be aware of it, but in other cases, they may not even know npd is, yknow, a thing!
4. Also keep in mind how other characters may react to their behavior! I’m sure you can tell from articles how a lot of people react to people with npd or those who exhibit symptoms of it... :’) That’s definitely an added challenge the character will face.
5. Finally, keep in mind npd tends to develop as a way to cope with not receiving proper amounts/kinds of attention and support while growing up! You’re definitely at a good first step of acknowledging that npd is stigmatized and expressing a desire to avoid contributing to that, which is great! If you can keep that understanding attitude and try to put yourself really into the shoes of someone who, say, wants acknowledgement and support from others (since they had little) or feels incredibly downstruck by the sudden lack of praise from others, I think that’s the best way to go.
As said before, people with npd are just regular people! I would personally say, if the character with npd is a major or the main character, these tips might be helpful! If they’re not as major of a character, you might not want to focus on the symptoms as much, if that makes sense? I personally think it’d be cool to have some deep-dives into characters with npd, but those would definitely be crossing into main character/significant character territory (if you want to write it well). Otherwise, if you write in the symptoms without much focus on the character motives, desires, vulnerabilities, trauma/past conflicts/etc., they might fall a bit short and wind up not being a great writing piece on a character with npd.
I hope this helps some! I’m not a professional writer by all means, it’s just a hobby of mine! And the points here are just what I focus on in my writing personally, and what I’d love for people to explore if they want to put a character with npd into their stories! ^^