Here’s a walkthrough/demo of how Threads Bookmarker works:
Go to this page (on mobile).
Install it as a PWA (Progressive Web App) — go here for instructions on how to do this.
Below any post or comment you like, tap the share icon.

Tap ‘Link’.

Return to Threads Bookmarker.
Tap ‘BOOKMARK’.

Tap ‘Paste’.

Done!
You’ve just bookmarked your first post! 🙌
Simply tap ‘VIEW’ to go to the original post on Threads or the trash can icon to delete it from your bookmarks.
Enjoy 💓