I THINK..... at the top/right of the thread you should see "Thread Tools" "Search Thread" and "Displey"
If you click on the little down arrow beside Thread Tools, a drop down box comes up, you click on subscribe to this thread. Then another screen pops up that has two things... "notifications" and "Folder"
Click on folder to add to "Bookmark" or "Subscription". You can then find those under your profile or settings.
Some how you can add and name your folders. I did but dont even remember how I did it!
Best way it to just play with it and learn by trial and error! Good Luck