How to apply CSS styles?

To apply CSS styles to your HTML documents, you can use one of the following methods:

Inline CSS: Add styles directly to an HTML element using the style attribute.
```
<h1 style="color: blue; text-align: center;">Hello World</h1>
```

Internal CSS: Include a <style> tag within the <head> section of your HTML document.

<head>
    <style>
        body {
            background-color: lightgray;
        }
        h1 {
            color: green;
        }
    </style>
</head>

External CSS: Create a separate CSS file (e.g., style.css) and link it to your HTML document.
```
<head>
    <link rel="stylesheet" href="style.css" />
</head>
```
In style.css, you can write your CSS rules:
```
body {
    background-color: lightgray;
}
h1 {
    color: green;
}
```

Choose the method that best fits your project needs. External CSS is generally preferred for larger projects as it keeps your HTML clean and separates content from presentation.