Core is a project-based software, so you must have a project to create an invoice. However, for small consulting gigs, you can create a generic project in Core and have your company be the client for that project. For your consulting work, create a phase for each client you provide consultations to. When you create manual invoices for each phase, you can edit the invoice to reflect your client's information, such as the billing address. Though you cannot associate multiple phases with a single manual invoice, you can still associate a manual invoice to the top-level project. Check the Core Help for more information on creating manual invoices.