I showed how to do this in the video, but for people (including myself) who don’t enjoy watching an instructional video, I figured I explain how to bookmark a post. A the bottom of every post are three dots between the link and reply buttons. Clicking those dots expands a menu that includes a bookmark icon. There is also a bookmark button at the bottom of every thread. Clicking or tapping on that brings up the “Create bookmark” dialog box:
You can ignore all those options and just save. But if you want to be reminded in the future or add a note to remember why you bookmarked the post, the options are there. Later, you can find your bookmarked posts in your profile or in the Bookmarks tab.