On the form view of an account journal, in the first tab, there is a many2one link to the sequence. When you create a new journal, you can keep this field empty and a new sequence will be automatically created when you save the journal.
Upon module installation, all existing journals will be updated with a journal entry sequence (and also a credit note sequence for sale and purchase journals). You should update the configuration of the sequences to fit your needs. You can uncheck the option *Dedicated Credit Note Sequence* on existing sale and purchase journals if you don't want it. For the journals which already have journal entries, you should update the sequence configuration to avoid a discontinuity in the numbering for the next journal entry.