Just use a 'bug report' template when creating a new issue.
The process of submitting a pull request depends on what kind of changes you have in it:
You can open your PR directly against a master
branch.
Open your PR against the dev
branch of the repository with a meaningful message in the description.
First, open an issue from 'feature request' template or describe in the existing issue what idea you are trying to implement – probably it needs some tuning to be more aligned with the whole library design and vision. After a design discussion, you can open a PR against the dev
branch.
If you have created a new serialization format, you can keep it in your repository to be able to perform proper maintenance and to have a separate release cycle. You can submit a PR to the list of community-supported formats with a description of your library.
You can find all the instructions here