To create a partial invoice, click on Create > A partial invoice when you're in a job.
The amount invoiced will be calculated based on the progress you defined for the job (50% in the screenshot above) or on the time you already spent on the job if you selected the 'Time-based invoicing' option for the job (see the article about the time tracking functionality).
You can create as many partial invoices as you like for a job. If you already invoiced 50% of the job and then continued to work on it, the next partial invoice will take into account the difference between the new progress and the 50% already invoiced (if the new progress is 70%, the second partial invoice will amount to 20% of the job (70%-50% already invoiced)).
When you create a partial invoice, the status of the job remains unchanged (In progress) because it's not finished yet. In the job, you see a message indicating how much has already been invoiced for this job.
The partial invoice appears in the list of invoices as any other invoice, and it'll be the same for all the next partial invoices for the job.