$ bzr clone lp:openerp
$ cd openerp
$ ./bzr_set.py -r tag:5.0.3 ../5_0_3
$ cd ../5_0_3/
Open one terminal for the server
$ cd server/bin/
(make sure you have a "terp" user on your system, and that you're logged as this user, and that there is a "terp" database in your PosgtreSQL server)
$ sudo su terp
$ ./openerp-server.py --db_user=terp --db_host=localhost
Open another terminal for the web client
$ cd web/lib/
$ ./populate.sh
(this will install all the required missing libs)
$ cd ..
$ ./openerp-web.py
Go with your browser to http://localhost:8080/. There you are.
Creating the database and all that is complicated. Have a look at
this other article to understand the process.