Using Subclipse:

1. Select the project you wish to create the patch from.
2. Select the Team → Create Patch ... context menu action.
3. Select "Save In File System" and pick a suitable file name.
4. Click Finish.

Alternatively, from the command line, in the project directory you want to create the patch from,

svn diff > ~/plugin.patch

The resulting file can be mailed (at the moment, please mail it directly to Miles Sabin) or attached to a Trac ticket, and applied later using the Team => Apply Patch ... project context menu action or the patch(1) command line tool.

If you're submitting a significant patch you'll have to complete a  Software Grant and Individual Contributor License Agreement and return it to EPFL. This is an unavoidable piece of bureaucracy which protects both you and EPFL. The document itself contains more information, and also details of where to send the completed copy. Small patches which fix a bug or provide a minor enhancement might be accepted without a CLA being completed.