We use issues for asking questions (open one here!) and tracking bugs and unimplemented features, and pull requests (PRs) for tracking and reviewing code submissions.
Before submitting a PR
Consider opening an issue to talk about it. PRs without corresponding issues are appropriate for fairly narrow technical matters, not for fixes to user-facing bugs or for feature implementations, especially when those features might have multiple implementation strategies that usefully could be discussed.
Our issue templates might help you through the process.
When submitting PRs
Please fill in the pull request template as appropriate. It is usually helpful, it speeds up the review process and helps understanding the changes brought by the PR.
Write clear commit messages that start with a one-line summary of the change (and if it's difficult to summarize in one line, consider splitting the change into multiple PRs), optionally followed by additional context. Good things to mention include which areas of the code are affected, which features are affected, and anything that reviewers might want to pay special attention to.
If there is code which needs explanation, prefer to put the explanation in a comment in the code, or in documentation, rather than in the commit message.
For pull requests that fix existing issues, use issue keywords. Note that not all pull requests need to have accompanying issues.
Assign the review to somebody from the Core Team, either using suggestions in the list proposed by Github, or somebody else if you have a specific person in mind.
When updating your pull request, please make sure to re-request review if the request has been cancelled.
Focused commits or squashing
We generally squash sequences of incremental-development commits together into logical commits (though keeping logical commits focused). Developers may do this themselves before submitting a PR or during the PR process, or Core Team members may do it when merging a PR. Ideally, the continuous-integration tests should pass at each logical commit.
Review and merge
Anyone may submit a pull request, and anyone may comment on or review others' pull requests. However, one review from somebody in the Core Team is required before the Core Team merges it.
Even Core Team members should create PRs for every change, including minor work items (version bump, removing warnings, etc.): this is helpful to keep track of what has happened on the repository. Very minor changes may be merged without a review, although it is always preferred to have one.