[TYPO3] applying a bug patch?

[Michael Stenskjær Christensen]

I've recently posted this question in the commerce list, but with no  
response, so now I'll post it in (what I take to be) its proper place..

Basically: how do I apply a bug patch to typo3?

There is this short guide to patches and diffs here: http://typo3.org/development/bug-fixing/diff-and-patch/
  - but I dont really know how to apply it anyway. the guide says:
"Patch your source code using a diff file:
cd typo3_src.new/
patch --dry-run -p1 < ../bug_1234.diff"

But (according to the terminal) the typo3_src.new/ is no directory
(which is already obvious to me) - so the question is: how do I gain
access to the site via the terminal? Should I write something in front  
before the first command? And is that the exact directory to use?
A terminal noob needs help here: how do I apply the patch, step by step?
I don't know much about it, so I can't supply you with much  
information until you ask me..

Any help will be greatly appreciated
- Michael S. Christensen