First, you have to setup the remote of the repository you want push into.
Actually, you want make changes in your fork only, so go in the root directory of the package (e.g.
vendor/package/name/) and run :
git remote add origin https://github.com/clytemnestra/JMSTranslationBundle
Then, just make changes and push when you are done.
When you want make your PR, just go in your repository (or in the original) and hit "Create a pull request".
It will automatically propose you to compare your own branch with one of the original repository.
Select the good branches and submit the PR.
Try to be as clear as possible with the reason of your PR, and if there is issues related to, just reference them in the message of your PR.
Note : To make a clean pull request, keep your changes in one commit.
To do this, see git rebase and squashing
Note 2: Commit only the changes you make on the files corresponding to the bug or feature you are working on. All other files should be intact, even if test suites doesn't pass, you don't have to fix them.
If you got an error like
origin remote already exists , run the following :
git remote set-url origin https://github.com/clytemnestra/JMSTranslationBundle