A journal is just like any other Odoo model or class.

You can create an instance of a model (record in a class/table) three ways in your own module:

• Via code

• Via XML

• Via CSV

Each option does exactly the same thing - create  a record in the database.  No matter how it gets there, the result is the same (even when added via the UI).

Code:

vals = {
        'name': 'The Name',
        'code': 'THECODE',
        ...
}

self.env[account.journal].create(vals)

More at https://www.odoo.com/documentation/10.0/reference/orm.html#common-orm-methods

XML:

Supply an XML file that imports a record into the database:

<odoo>
    <data>
   
        <record id="your_id" model="account.journal">
            <field name="name">The Name</field>
            <field name="code">THECODE</field> 
            ...             
        </record>

    </data>
</odoo>

More at https://www.odoo.com/documentation/10.0/reference/data.html

CSV:

Supply a file called 'account.journal.csv' in your module with the fields and values you want to import.  You can export an existing Journal to see the schema for the CSV file:

"id", "name", "code", "company_id/id", ...
"your_id", "The Name", "THECODE", "base.main_company", ...

More at https://www.odoo.com/documentation/10.0/reference/data.html#csv-data-files

Note: Since a Journal also requires a sequence, you should also create one of those, before creating the Journal, so the Journal can refer to it.