As per How do favorite questions work? you simply mark the question as a favourite.
A question is marked as a favorite by clicking the star beneath the vote counter:
A number beneath the star shows the number of users who have favorited a specific question. Marking a question as a favorite basically says that a particular user feels like watching that question. It's completely separate from the voting system and has no impact. However, having a question marked as a favorite by enough users does make you eligible to earn certain badges.
Marking a question as a favorite allows you to easily check back and see when updates are made to the post, and can function as a simple bookmark for questions you might want to reference again in the future.
Any question you favorite will be listed in your user profile under the tab "favorites". This lets you find it easily when the question may otherwise get lost.
The favorites tab on your profile has a counter indicating how many of your favorite questions have changed since you last viewed that tab. When you click on the tab, recently changed questions will be highlighted. "Changed" includes comments on or edits to the question and new or edited answers. This is a way to get updates on questions that you yourself didn't ask.
There is no limit to the number of favorites you pick. Furthermore, you can retract a favorite at any time by just clicking on the star. There is no limit to the number of times you can retract or reinstate a question as your favorite.
You can see the favorites of other users in the user profile page, and which users have favorited a specific question by using this query on the Data Explorer site, for example list of users who favorited this very question.