Since ReactOS does not have a definite maintainer for each section of the code, you should search for a bug report matching the issue you'd like to fix, and perhaps file a new bug in Jira and attach your patch. Set the severity level to major to draw the developers attention to the patch. If you are unsure about something, talk to a developer in the #reactos IRC Channel.
Test your patch
Thoroughly check that your change works as intended.
Test your changes with the latest code from SVN. Something may have changed in the latest revision that either breaks your change or causes your change to break something else. Make sure that ReactOS can still boot and that any application or system that may be affected by your change still runs properly.
Preparing the patch
Make sure that your patch is against the latest code from SVN. Where you have made changes to an existing source file, you have to use the diff function of your SVN client to obtain a file that contains only the changes that you have made. If possible, start the diff in root directory to preserve path information of changed files. Please create one patch file only, even for multiple files (new ones as well) as long as they belong to one change.
Also make sure that you have installed an SVN client. The page Subversion - Access the sources gives you more information about getting an SVN client for your operating system.
Then read the section Creating a patch/diff file of the usage page for your SVN client.
All these usage pages are listed here: Subversion - Usage of the SVN clients.
Attaching the patch to the bug report
If your changes are all in one patch file, you can simply attach the patch file to the bug report. Don't forget to tick the check box patch when attaching the file to indicate that this is a patch file.
If your changes can't be stored in just one patch file (for example binary files can't be stored in patch files), create a ZIP file containing your new files and attach it to the bug report.
Ensure that the title of the bug report summary is prefixed with the word "PATCH:" (no ").
Important: provide your real name and e-mail address. Patches from unknown authors are not accepted!