The Yamaha YDP-300 is part of Yamaha's Arius series of digital pianos. So, another name or way to refer to it might be "Yamaha Arius YDP-300." However, manufacturers often have various models and regional naming conventions, so it's always a good idea to check the official website or product documentation for any alternate names or designations specific to a particular region or market.
Yamaha's website didn't have the YDP-300 listed... we ended finding that the manual for
YDP-160
YDP-140
https://usa.yamaha.com/files/download/other_assets/0/333800/ydp160_en_om_c0.pdf
Matched the pictures. But it had no info on how to open it up! Here is how to open it up.
Laying on your back under the piano there is a screw on either end angled at a 45 degrees in a steel plate. Loose these screws several turns (they don't remove completely). Then it will open!
Most of the rest of the disassembly to fix any sticky/quiet or broken keys is just all the screws you can see.
Some cables should be unplugged as needed. Also to lift up the rack of keys there are 3 screws underneath (laying on your back, looking upwards) that also need to removed.
In our case, if a drink or ramen or something else is spilled on the keys, you have to get pretty far through the layers of screws to get to the circuit boards under the keys themselves. It is broken up in to 3 panels with a bunch of visible resistors. After you get access to the boards, wipe it off with rubbing alchohol, and then also clean the black rubber contacts with rubbing alcohol.