To create a partial invoice, click on Actions > Create partial invoice when you're in a job.
The amount invoiced will be calculated based on the progress you defined for the job (see https://help.lsp.expert/en/articles/747166-can-i-mark-the-progress-of-my-job-somewhere) or on the time you already spent on the job if you selected the 'Time-based invoicing' option for the job (see https://help.lsp.expert/en/articles/728072-how-does-the-time-tracking-feature-work).
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).
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 you already invoiced for this job.
A partial invoice will appear in your list of invoices as any other invoice, as will all the next partial invoices for the job.