Applicants:

Post the link to your YC 120 video in a top-level comment. See [2] for instructions. Minimize extraneous text: YC 120 applications don't leave a lot of room for additional text anyway, so it's best if your video can stand on its own.

Critiques:

Attach critiques as second-level comments under the videos you're critiquing. If you need additional guidance on how to critique, I recommend Mary Robinette Kowal's excellent infographic on the topic [3].

Replying to Critiques:

Reply to critiques only for clarification. Avoid defending yourself; arguing doesn't accomplish anything. I recommend [3] if you're struggling to figure out how to interpret feedback.

Others:

Avoid discussion of YC 120 itself; there have been other posts for that and you can start a new one if you want to discuss it more.

Good luck everyone, and don't forget to enjoy the journey!

[1]: https://news.ycombinator.com/item?id=19028916

[2]: https://blog.ycombinator.com/yc-120/

[3]: https://www.patreon.com/posts/manuscript-arent-11552026 (while intended for novels, much of it is generally applicable)