You have two options to easily create an invoice:
When you're in the job, you click on Create > An invoice.
When you're in the list of invoices, you select the job(s) you want to invoice and you click on the arrow > Invoice selected jobs. It also works if you select jobs for different clients (in that case, one invoice will be created for each client).
Whatever the option you choose, you'll arrive in the invoice screen, where you can edit the details of the invoice and jobs, add lines and jobs, create a PDF document, send the invoice by email, etc.
There is a third option to create an invoice, but it might take a bit longer. You can create an invoice "from scratch" by clicking on Add an invoice when you're in the list of invoices. You then arrive in a blank invoice where you can add lines and jobs (by clicking on the Add button).