You have two options to easily create an invoice:
When you're in the job, click on Actions > Create invoice.
When you're in your list of jobs, select all the job(s) you want to invoice and click on Actions > Create invoice(s). 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 and the billing options of the invoice, add lines and jobs, create a PDF document, send the invoice by email, etc. (To know how to send your documents through LSP.expert, see this article: https://help.lsp.expert/en/articles/735154-can-i-send-my-documents-directly-from-lsp-expert-to-my-clients.)
