To associate a Client with a project go to Projects>>Project Name and click the blue edit icon to the left of the Client field (top left). Then follow the directions (here).

To associate a Client with an Invoice go to the Invoice Preview and Setup Page. Invoice Manager>>find the Invoice in the list>>click the blue Edit icon at the left hand end of the row. An administrator may have to set the invoice status to Open to enable this action. Then follow the directions (here).