I have a problem with invoicing my clients. 

I've opened projects for each of my clients, for every team that I have separately ( X company design, development, video and content), ( Y company design, development, video and content) etc and I opened many tasks in each of them and my team has worked on them during the last month, and keeps working on the new ones and now I want to send to each of my customers invoices regarding their projects (tasks) but I don't know how to do that, although I have watched the odoo tutorials, but they were not solving my problem, as in order to invoice your customers, first you need to open a sales order which creates a new project with new tasks, but I'm planning to invoice them for the projects I already created and tracked. Has anyone dealt with this issue? How did you sent your invoices?