Let's create a project called plone.app.myproject:
$ mkdir plone.app.myproject 

$ cd !$

$ git svn init -s http://server.com/!$
The last command assumes that plone.app.myproject/trunk, plone.app.myproject/branches and plone.app.myproject/tags already exist in the svn repository.
$ git svn fetch

$ git add .

$ git commit -m 'initial import'

$ git svn dcommit
That's it. At this stage the master branch is reflecting the remote trunk.