From WxWiki

Prerequisites

You need a local copy of wxWidgets CVS repository. To get this, use the following CVSROOT:

<strong>:pserver:anoncvs@cvs.wxwidgets.org:/pack/cvsroots/wxwidgets</strong>

(in TortoiseCVS, right-click somewhere in explorer and choose 'CVS Checkout...' and do a copy-and-paste of the above CVSROOT in the CVSROOT textbox control) to checkout the module 'wxWidgets'. Then, when asked for password, type in '<strong>anoncvs</strong>'. Then just sit and wait: wxWidgets' full repository is quite big; however, you can also checkout only the portion you have modified: do a checkout of 'wxWidgets/path_to_the_file_i_need_to_send_a_patch_for'.

The patching process

Download the diff utilities for windows at http://gnuwin32.sourceforge.net/packages/diffutils.htm. Using an MSDOS prompt go to the folder where you keep your modified wxWidgets tree (e.g. "wxWidgets") and the CVS one (e.g. "wxWidgetsCVS") and then type in:

 diff -ru2 -x"CVS" -x".#*" -x"*.orig" -x"*.rej" -x"*~" wxWidgets\mymodifiedfile1.h wxWidgetsCVS\theoriginalfile.h

this is the fastest way to create a patch since it requires to use CVS only to checkout the current wxWidgets CVS module and does not use it for the actual diff (on my PC, when I try to use "cvs diff" with the wx tree, I can wait for ages and nothing happens: cvs hangs !).

If your CVS is faster (or better, your connection is faster !), then you can also use the command:

cvs -d:pserver:anoncvs@cvs.wxwidgets.org:/pack/cvsroots/wxwidgets diff > mypatch.patch

from the prompt placed in the same folder where you checked out the wxWidgets tree.