Must have 2 SQL servers, 3 if you'd like a witness server:
2 servers need to have the same versions of SQL running on them.
The primary database will need to be in Full Recovery model (right click on db->properties->options)
-Full backup the Principal DB
-Create a database with the same name from the Principal SQL Server on the Mirroring SQL Server, then restore the backup on the Mirroring SQL Server with the option to Overwrite the existing database checked and RESTORE WITH NORECOVERY option.
-Check mirror database should show as being in 'Restoring' mode
-Full backup the transaction logs and restore (no recovery option switched on!)
-You may need to repeat last transaction log if failing as it needs to be as close as possible in time.
Right click the principal DB and choose 'Mirroring'
-Now choose 'Configure Security' to run the setup wizard.
-Run through the wizard.
-Then start the mirror
You should be good to go!
If you get Error:
The mirror database, “YourDatabaseName”, has insufficient transaction log data to preserve the log backup chain of the principal database. This may happen if a log backup from the principal database has not been taken or has not been restored on the mirror database. (Microsoft SQL Server, Error: 1478)
Then make another transaction log backup and restore it again, then restart mirroring.