I got it right, but only because I thought you had to link the mirror to the principal first, and then the principal to the mirror (which starts the mirroring session).
Establish a Database Mirroring Session Using Windows Authentication (Transact-SQL)
(points 4 and 5)
Are you sure a log backup is needed? Quoting from BOL:
Typically, you need to take at least one log backup on the principal database. However, a log backup might be unnecessary, if the database has just been created and no log backup has been taken yet, or if the recovery model has just been changed from SIMPLE to FULL.
Prepare a Mirror Database for Mirroring
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MCSE Business Intelligence - Microsoft Data Platform MVP