Saying Bli Neder

If someone doesn’t want a certain action of theirs to become a neder, is it enough to just have in mind “bli neder” or the like, or does it have to be verbally stated?

A – You have to say it and even once you say it you must do whatever you promise, the saying only helps that it doesn’t become a neder
Q – I meant if its like a chumra and the person didnt initially accept it by words. Same din applies?
A – Can you be more detailed
Q – Like washing with kli after restroom
A – If you just started to wash with a keli you can rely on the Atara we do before kipur