Take their defining characteristics.
We all know the person I'll be using for an example.





Tom Holland, the best Spider-man! Everyone I know loves him. Well, let's take a look at his face. If we're going to draw him, we gotta find out what makes him, him.

Brow Area, Lips, Nos


Listed are what I see first. So, that's what I'd draw first.



Eyes, Glasses, Hair

This is where you get to express freedom. Because fictional characters don't exactly have realistic proportions, you can almost always draw them however you wish.
Let's take a book character. If you grew up from 1995-2005 like I did, you probably read Percy Jackson. Or you're younger/older and read it, you count too.

In the novels, the author doesn't really describe Percy because Percy is our point of view. But we do get hints. He has his father's eyes, which are sea green. He has black hair. And that's pretty much all we know.

You can draw any character with those features and call him Percy Jackson but it just doesn't... look like him. So what else can we do?

BOOM

CAMP HALF-BLOOD T-SHIRT, THROW HIM IN THIS AND WE ALL KNOW WHO HE IS

Clothes and accessories and little details are super important! If you're drawing a headshot of Percy, slap on a bit of orange somewhere! Add a trident in the background!

But... what about your own characters?

Well, that's easy. Snag a reference image from Google for their face, and add in the little details.

Okay, let's take my character Ali. I've only drawn her once, and it didn't exactly work.

If I just draw her without a reference, she'll look strange, and almost plastic. But what if I gave her features from an existing celebrity?



This beautiful woman is Halle Bailey. She looks badass, doesn't she? I totally see my character in her. In fact, I see Ali's eyes.

So let's give Ali those gorgeous eyes.

Okay, what about the rest of her features? Well, I do the same thing. I search the Internet for certain features, and I slowly add them on until I create a completely different character.

Now let's talk about science.

You can't create a full mental image of a person. Once upon a time I tried to do this with one of my characters from an abandoned project, and uh, I kept forgetting. No matter how many times I described his features to myself, I just couldn't get them right.

Because I've never seen anyone with those specific features. And also because I have Aphantasia, but that's a different subject.

But because I was able to mix together a bunch of badass women in a stew, I can draw Ali without any worry. And that's all it takes to draw portraits.