It looks cool and I want to learn how.
It’s a filter in Photoshop as far as I know.
It looks shit and cheap anyway and everyone overdoes it.
don’t do it.
There’s a Channel tab next to your layers in Photoshop, get there, pick one of the color (Red looks better) and translate it 1px to the right and 1px up. That’s how I do it. I kinda like this effect but apparently not everyone does so I suggest you don’t use it.
It only looks good when it’s used very subtly and when it makes sense.
Or, of course, for any kind of optical device (camera lenses, rifle scopes, binocs, magnifying glasses, mirrors) or reflections from placid muddy water, in which cases you absolutely should use it.