There's no way to do this besides restoring the Content Database to
another SharePoint farm and using STSADM -o backup to back up the Site
Collection you are looking for.

Step/step instructions are available @ Technet:

technet.microsoft.com/en-us/library/cc719751.aspx

Backup/Resrote site  using DPM : -->
technet.microsoft.com/en-us/library/cc770235.aspx

I think this is not totally accurate.

The issue is whether you want to replace everything that was in the
content database  with the version in the backup, or whether you want
to selectively restore  some parts of the content  data.

Selective restore requires your method.

But if the entire content database need to be rolled back, just
detach the old content database and attach the backup version.

Your instructions are also for the situation where you are trying to
create a second copy of some site. The reason you can't just attach a
second copy of some content database to a SharePoint site  is because
the sites in the duplicate content database would have the same GUID
as the original sites. That's the reason to restore it to another
farm and use STSADM to copy it back to the original site. STSADM -o
restore creates a new GUID.