svn co http://svn.luci.subsignal.org/luci/trunk
and change to that directory:
cd trunk
Edit the files you want to change. If you add some new files you need to add them to the svn tree:
svn add <dir/files>
Where are the directories/files you have added. Its possible to specify multiple files/directories here.
To check if your changes look ok first do:
svn diff <dir/files>
and check the output. Again you can specify multiple dirs/directories here.
If everything looks like expected save the patch:
svn diff <dir/files> > ./mypatch.patch
Use the Ticket system to submit your patch.