Apply Patch

With TortoiseMerge:

1. Find and open an existing SVN repo directory
2. Create a new directory named "merges", if it does not exist already
3. Copy the file onto which you want to apply the .patch file
4. ADD and COMMIT to the svn repository before you continue to the next step
5. Right click on merges and choose Apply patch...
6. Double click the file from list
7. The patched file with diff is displayed on the right pane
8. Click on that pane and hit Save or export with File->Save As...

Alternative screeny if you Open from TortoiseMerge. In the screeny below, directory refers to the "merges" directory mentioned at step 2 above:

Screenshot of WinMerge GUI:

Note: You can first preview what your patch will do:

First the stats:

git apply --stat a_file.patch

Then a dry run to detect errors:

git apply --check a_file.patch

Finally, you can use git am to apply your patch as a commit. This also allows you to sign off an applied patch.
This can be useful for later reference.

git am --signoff < a_file.patch 

See an example in this article:

In your git log, you’ll find that the commit messages contain a “Signed-off-by” tag. This tag will be read by Github and others to provide useful info about how the commit ended up in the code.