Goal: 
Manually test a patch or merge request that has been proposed as the fix for an issue in the Drupal core project, or in the project for a contributed module, theme, or distribution.
Detailed steps: 
  1. Set up prerequisites: Log in and either be ready to set up an Online testing site, or else have a Local development site and Git set up, from Common Prerequisites for Contributors. (These steps assume you will be using an online testing site.)
  2. Choose a project to work on, either Drupal core or a contributed module, theme, or distribution that you are familiar with.
  3. Find the issues for the project (example: issue list for Drupal core).
  4. Filter the issue list to status "Needs review"; for Drupal core, use the advanced search to filter to status "Needs Review" and the special tag "Needs manual testing" (or another special issue tag that fits your skills/interests, such as "Needs design review").
  5. Choose an issue to work on. Read the issue and make sure you understand the problem, what versions and browsers it applies to, and the proposed resolution. If not, choose another issue.
  6. Create an online testing site to verify that the problem still exists -- see the "Using online Drupal testing sites" section of the Setting up a test environment page. In the Project field, select the project you are testing (Drupal Core or the contributed module, theme, or distribution). For the version, select the latest development branch that the issue applies to. (Branches are listed at the bottom of the version list, and end with ".x".)
  7. Verify that the problem described in the issue exists on your test site. If you cannot reproduce the problem, add a comment to the issue describing what you did, the environment (versions of Drupal core and modules, browser, etc.), and what you saw. (See Creating or updating an issue report for details on how to comment on or update an issue.) Then find another issue to work on.
  8. If there is not already a screenshot of the problem behavior on the issue, take one or more screenshots and save them locally with descriptive names (example: without-patch-views-admin-page.png).
  9. Create a new online test site to test the patch or merge request:
    • If the issue has a merge request, there should be a link in the merge request area to a preview site. See Using live previews on core merge requests for more information.
    • If the issue has a patch file, you can use the same steps you used previously to set up a new online test site, but also include the patch file URL in the setup form in the "Advanced" area.
  10. Follow the same steps that you used on the first test site to test whether or not, with the patch or merge request applied, the problem still exists. Take one or more screenshots and save them locally with descriptive names (example: with-patch-views-admin-page.png).
  11. Try related tasks on your new test site to verify that the patch does not introduce new bugs.
  12. If the user interface or UI text has changed due to the patch, verify that the new UI follows the Drupal project's user interface standards and in particular, interface text standards. If the graphic design has changed, verify that the new graphic design fits with the design ideas of the theme you are reviewing.
  13. For issues involving changes to the user interface, test using different core themes (unless you are working in a contributed theme project) and make additional screenshots if the behavior is different.
  14. If the patch or merge request for the issue involves any CSS or JavaScript changes, repeat the test with other browsers (you don't need to take additional screenshots unless the behavior is different in another browser). See the list of supported browsers.
  15. Write a comment on the issue with the results of your testing. Explain what steps you followed, what themes you tested in, which browser(s) you used, and which versions of projects you used.
  16. Upload the screenshots you created to the Files section. Optionally, embed them into your comment using img HTML tags to illustrate what you saw.
  17. If the patch does not fix the issue, or if it caused new bugs, set the issue status to "Needs work."
  18. If the patch does resolve the issue and has been tested in all supported browsers and/or all core themes, remove the "Needs manual testing" tag if it is present.
  19. Click Save to save your comment and issue updates.