You need to have access to sale model to be able to create a record 

In your create method of your custom model you can call the below

self.env['sale.order'].create(vals)

vals: a dictionary for all sale.order and sale.order.line values, You need to pass all required fields and other fields you want.

For example:

vals= {
    'partner_id': self.partner.id,
'order_line': [(0, 0, {
        'product_id': self.product.id,
'product_uom_qty': 1,
'price_unit': 192,
'discount': 74.246,
})]
})