Tap the bottom right icon (person) to go to your profile, then press the bookmark button in the top left. You will need to sort by something that isn’t time-based (“Top 6/12 hours” only shows bookmarks from the last 6 or 12 hours.) “Active” works for me.

Click on your little person icon in the bottom right corner, then click on the bookmark icon in the top left corner.

Follow-up question: how do I unbookmark things? I don’t see the button anywhere on Android. I can see the green bookmark icon when I swipe the post like halfway across the screen, but that doesn’t seem to remove its bookmark.