Before you can apply this principle, there is one thing that you have to know: how to recognize bad ideas. Thankfully, there are a lot of software design principles that help clue you in on what is a bad idea, and lead you to saying "no" when it's truly needed. For example:
If the implementation of the feature violates the laws of software design (for example, it's too complex, it can't be maintained, it won't be easily changeable, etc.) then that implementation is a bad idea.
If the feature doesn't help the users, it's a bad idea.
If the proposal is obviously stupid, it's a bad idea.
If some change doesn't fix a proven problem, it's a bad idea.
If you aren't certain that it's a good idea, it's a bad idea.
Also, one tends to learn over time what is and isn't a good idea, particularly if you use the above as guidelines and understand the laws of software design.