Does the PR miss info required in the Pull request template? Example: #6395. Add labels 'Needs revision' and 'Needs response from author'. Add a response like:
Hey @author, thanks for sending the pull request. Can you please add all the info specified in the template? This is necessary for people to be able to understand and review your pull request.
Does the code style match the Style guide, especially consistency (formatting, naming) with the rest of the codebase? If not, link to the style guide and add the label 'Needs revision'.
Does the pull request add a completely new feature we don't want to add to the core and maintain? Ask the author to release it a separate npm module and close the PR. Example: #2648.
Does the pull request do several unrelated things at the same time? Ask the author to split it.
Is the PR old and need rebasing? Ask the author "Can you rebase please?" and add the label "Needs response from author".
Is a PR waiting for a response from author (usually has label "Needs response from author") and there's no reply in last 30 days? Close it with the bookmarklet. Examples: #3066, #1099.
Is the PR old and waiting for review? Review it or cc someone who might be able to review.