User is trying to figure out a way to set up a recurring invoice, but be able to edit the descriptions of the line items that make up that invoice prior to its monthly recurring billing.
You can set up recurring invoices by either memorizing an existing invoice or by setting up automatic billing for a project. A recurring invoice does not allow you to specify the individual items or invoice amount because it’s based on an existing invoice. There is no way you can edit the description of the line items.
In case of automatic billing, the frequency of billing is fixed for a project, but the amount is based on the logged time and expense entries, which can vary each billing cycle. You might want to use this option instead of the recurring billing option. Check the Core Help for details on Automatic Billing.