If its a Wii u do the same thing, if not just look up "3ds Shoulder Button replacement" and watch a few videos, some have a link to purchase shoulder buttons, compare prices before buying. Than replace the buttons, get the required tools usually just a screwdriver and tweezers.

If you can't do it, its actually simple so it should be easy, but if you can't then send it to a trusted repair shop, or send it to a good rated local repair shop. They can do it, its cheeper to DIY.

Avoid using tweezers for ribbon cables, at all costs. Use a blunt thin piece of plastic to remove it from the board.

Basically just follow the tutorial you chose.

thanks