You can set options for invoices and also create them from the Projects > Billing > Options screen. If a project has the Slips Require Approval option selected, then slips must be approved before they can be billed.
1. Specify your invoice preferences on the Projects > Billing > Options screen. 
2. Click the New button to create an invoice. The Invoice Dates dialog displays.
3. Select Start and End date for the range of time entries to be billed. You can also select an Invoice Date to appear on the invoice.
4. Click Continue. The draft invoice displays. If a draft invoice has been previously created for this project, you will receive a prompt to confirm deletion of the previous one. Click Delete to proceed.
5. To redisplay a previously created draft, click View Draft. To finalize this invoice, click Finalize Draft. This invoice will appear on the Invoices tab of the Projects, Contacts and Billing screens. 
6. To review the invoice details, select the invoice row and click the Edit icon.
7. To reverse the invoice so that it can be rebilled, click the Delete button on the Invoice Details screen. Time slips will be released back to the unbilled status.