If you don't use the setting to automatically send the invoice with the submission email, you have to send the invoices yourself.
You can do that for one participant or in bulk.
Send invoice to one participant.
Edit the participant via the people list in the participant module.
Go to the invoices tab:
Use the 'Preview invoice' button to see the invoice on your screen before you email it.
When you press the 'Send invoice' button, you will first get a confirmation screen:
If you confirm, the email will be send. Pressing 'No' will cancel sending the invoice.
After you have sent an invoice to a participant, the screen will look like below.
You can edit the invoice to make small changes with the edit icon
You can recreate the invoice
Or you can view the invoice as a PDF
Send the invoices in bulk