The idea will be first proposed and discussed among the experts who are familiar with the subject. Then the developers have to write the code for the proposal and publish it for review before it is deployed in testing environment (a separate test network) to find its flaws and improve it further.
All these steps can be reviewed by the community too. But in the last step regular users could also test things if they duplicate the code and compile/run it on their own.
Finally after the bugs were fixed and if the change is deemed worthy it will be put to vote.