To make a tag, eg for a release, run a command line this checked-out head directory:
svn cp . https://svn.complete.org/offlineimap/offlineimap/releases/0.9.0
Subversion uses a "copy on write" method, so this does not actually use up disk space for a whole additional copy of the tree. Complete.org uses conventions as documented on the Subversion repository page for naming of releases and branches