I guess it depends on the art style. But generally, if you want to do it the hard way:

1. Get a drawing tablet. The ones where you draw directly on a screen are the best, but they are also way more expensive.
2. Get some powerful software that can imitate brushes. Krita is good.
3. Practice, experiment, watch videos and gather knowledge on both the art style and your software of choice. Copy pieces of art in the style you want to reproduce until you master it.

With an emphasis on the third point. Digital art done manually is not so different from traditional art - it takes practice.

Of course there are also styles you can reproduce using filters in GIMP or whatever. And you can do the above by drawing over pictures you’ve taken or borrowed for the purpose. It’s really a question with an unlimited number of potential answers.

The way I see it, there are, generally speaking, two main routes:

1. Draw the pictures yourself,
2. Write a computer program that can generate some sort of art.

Drawing requires lots of practice, as others have already pointed out. Writing a program takes skill, patience, knowledge and time, so it’s not that different really.

The main difference is what happens next. If you’ve made a picture, you have a single picture. If you’ve made a picture generating application, you can have unlimited pictures. The latter option comes with some pretty harsh limitations, because the range of possible pictures can be very limited.

Here’s an example of option 2: pattern collider

One useful program for this is called a web browser you can use to search for artists

Still trying to figure out Hades?