Create an issue with the changes you want to add
Wait for your feature to be approved
Fork the Project and make a branch with the features name
make a pull request from your project to the testing branch (pr's to any other branch will be removed)

Now your contribution is in testing where people can start using it as beta on their own servers and report back issues Once the testing phase is over and now reports come in the feature will be pulled to release from which it then gets its own downloadable release created

contribute with requests

if you want a certain feature in my plugin write an issue with the enhancement label you issue should follow this structure

Feature request

Name: how should the feature be named
Version: when would you like the feature to be implemented (this might differ from actual implementation)
Description: get creative and technical what do you want this feature to do. Your description even might contain pseudo code if you want to provide such

Your feature now can be reviewed by the community and be disscussed uppon and maybe changed to fit the plugin once it is approved it will be given a releaes milestone and its own feature branch from where it then will continue on the development proccess