Import the CSS file:
<link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/iconoir-icons/iconoir@main/css/iconoir.css" />
Here is an example in HTML:
The class must always be "iconoir-" and then the name of the icon. You can find the names of the icons here.
The icons are display: inline-block and default to the current font size. You can control this by adjusting the ::before styles of the element (which is where the icons are added as a mask).