Goal: 
Find an issue that has a patch (software fix) that needs to be updated to apply to the current software base, or "rerolled", and update the patch. This is important because sometimes a patch worked with code from the past, but changes to the code base make a patch not apply anymore. Re-rolling makes a patch that is essentially the same, but now applies to the current code base.
Skills required: 
Using Git
Background and reference information: 
Find the issues for a project
Creating or updating an issue report
Rerolling patches
Detailed steps: 
  1. Set up prerequisites: Log in and Git from Common Prerequisites for Contributors.
  2. Find the issues for Drupal core or another project that interests you.
  3. Click advanced search, and search for an issue with tag "Needs reroll". For example, here is a list of Drupal core issues tagged "Needs reroll".
  4. Make a comment on the issue saying you are going to reroll the patch. (See Creating or updating an issue report for instructions.)
  5. Reroll the patch, following the steps in Rerolling patches.
  6. If you are working in the Drupal core project, check locally that the patch or merge request passes the pre-commit checks, following the steps in running core development checks.
  7. Return to the issue page, and update it as follows:
    • Add a comment. Say that you rerolled the patch, at a minimum. Optionally, add details such as what conflicts there were, links to other issues that caused the patch to not apply, questions you had, etc.
    • Remove the Needs reroll tag
    • Set the Status to Needs review
    • Upload your rerolled patch in the Files area
    • Click Save to save your comment and changes