From Joomla! Documentation
Tortoise SVN is a simple solution for Windows users working with Joomla!'s SVN environment. You can use it to download a current copy of the development repository and to create, apply, and revert patches for testing with the Joomla Bug Squad.
Prerequisites: A local web host environment, such as XAMPP and administrator privileges on a Windows environment.
If you are using an IDE, you can usually perform these functions in that environment. Instructions for Eclipse are available. If you use Eclipse, Tortoise SVN is not required (although it can still be used along side Eclipse.)
Install Tortoise SVN
Navigate to the Tortoise SVN Site.
Download and install either the 32 or 64-bit option, depending upon your operating system requirements.
Create a local testing environment
Prerequisite:XAMPP or another local host environment.
In order to create and test patches, you need a current copy of the development code repository. Tortoise SVN can be used to connect to the development environment, download the current code base, and place it in your local development environment.
Create an empty folder for a new Web site within the Web sites folder (usually named htdocs) of your localhost environment.
Right-click the name of the empty folder and select SVN Checkout from the popup list.
Enter https://github.com/joomla/joomla-cms for URL of repository and press OK.
Wait while all files are downloaded from Joomlacode.
Update for the latest commits
Right-click the repository created above. Click SVN-Update.
Wait until all necessary changes have been made. Click OK to close the dialog box.
Apply a patch
Download the patch file from an Issue report on Joomlacode. (File tab on bottom of an issue report page.) Save it to your local environment.
Right click-the repository folder and select TortoiseSVN-Apply Patch from the Pop-up menu.
Locate the patch saved above, click open.
When "File patches" dialog box to appear, right click and select "Patch all."
Close the Dialog boxes and test the patch.
Revert a patch (remove it after testing)
Right-click the repository where you applied the patch. Click TortoiseSVN-Revert.
Select specific files or all files (bottom) to revert the patch changes. Press OK.
Wait until the Revert action is complete, then press OK.
Create a patch (after you fixed a bug)
Right-click the repository where you made changes to the code. Click TortoiseSVN-Create patch....
On the Create Patch Dialog box, select the specific file(s), or all files (bottom) that apply to the patch changes. Press OK.
Save the patch file to your local environment. There is no standard naming convention; many times developesr will named the patch NNNN.patch where NNNN is the issue number and 'patch' is the file extension.
Upload the file as an attachment to the Issue.
Note: always delete old patch files so that it's clear to testers which file to use.
Document your issue to include clear testing instructions.