The same way 90s kids learned to dress like earlier generations. We learned it from the movies and TV reruns. We learned to dress like 50s greasers from Grease. We got our 60s and 70s hippy fashion sense from Cheech and Chong.

Grungy Hacker chic: See movies like Hackers (duh), Strange Days, The Matrix, Fight Club, The Crow, Blade, The Fifth Element, Tank Girl, etc. Tight fits. (You can spot the squares in this aesthetic by their baggy tracksuit fits). Lots of dark and dirty retro futurism stuff. Deliberate splashes of vibrant colors if anything other than black. Lots of strange materials you wouldn't normally consider clothing. Did they literally pick that accessory out of the trash? Maybe. Eyeliner on everyone, even the boys, especially the sad boys. Big black boots.

For more normal stuff, see the fashions in Weekend at Bernies, Wayne's World, Airheads, Bill and Ted's Excellent and Bogus Journeys, Go, Mallrats, Chasing Amy, Clerks, Friday, Point Break, My Cousin Vinny, White Men Can't Jump, Bad Boys, Clueless, Empire Records, etc. Lots of baggy fits. Lots of flannel, usually layered over a T-shirt. Ripped Jeans. Mostly muted subdue colors, with occasional splashes of virbancy, like a loud tie on a brown suit. Big and often long hair on the boys (and no beards). Sneakers. Tracksuits. Typing this out, these styles seem way too real and not all that exaggerated.

And then very briefly, there was a flash of retro swing revival and everyone wanted to dress like Jim Carey in The Mask.

I guess younger imitators might try to throw all these styles in a blender and see what comes out. Flannel Goths. Bubblegum Neon Hackers. Zoot Track Suits. Ripped Jeans and literally garbage. No clothes at all, just rocking boots and their cyber deck/stim-suit like a princess from Mars as written by William Gibson.