The first step is to be sure your client has an existing profile.
Next, you'll also want to ensure you have added some invoice products in Invoice Set Up (My Settings > Settings > Invoice Set up > Invoice Prodducts)
Next, you can select "Invoices" from the main navigation at the top of the page. Once you're on the Invoice page, select "New Invoice".
Alternatively, you can go to your client's profile and select Invoices from the left side navigation. Then select "New Invoice".
Here you will select the client (if not already selected from the client profile), set the issue and due dates, and add a memo if needed. You can also opt to use an invoice template which will load any invoice items/settings onto this invoice once created to save you some steps/time. Then select "Create Invoice".
Select "Add" to add line items to your invoice. You can adjust the line item before saving if needed - such item description, quantity, and price. You'll want to select the check mark to save the line item.
You can add a discount to the invoice by applying a dollar amount or percentage amount. You can choose whether the discount is applied before or after tax, if applicable, by using the switch button next to the discount.
You can send the invoice as is or you can add a payment schedule on the invoice to break it up into installments.