RESTORE DATABASE blah FROM DISK = 'BACKUPPATH\DB_FULL.BAK'

WITH STATS, NORECOVERY;

RESTORE DATABASE blah FROM DISK = 'BACKUPPATH\DB_DIFF.BAK'

WITH STATS, RECOVERY;

Of course you'll also need WITH MOVE clauses if the new database has a different path.

And the database will be restored readonly, as you set it readonly before the diff backup.