It shouldn't be any different than any other site/collection: http://office.microsoft.com/en-us/sharepoint-designer-help/get-started-with-sharepoint-designer-2010-HA010370548.aspx#BMopensites
Have a great day,
- Proposed as answer by Daniel Trautman Thursday, June 14, 2012 5:02 PM
I would have to disagree. The only difference is that you paste in the URL of the site you are looking to modify.
1. (after opening SharePoint Designer 2010) click Open Site
2. Paste in URL (minus the home.aspx part of it)
3. Enjoy modifying your page
- Proposed as answer by Daniel Trautman Wednesday, October 24, 2012 9:36 PM