The Jetpack CRM Invoice Builder has many features, but still works out-of-the-box. You can even add online payments via Invoicing Pro.
This guide covers the basic use of Invoicing in Jetpack CRM.
Before you get started: Invoicing Settings
The Invoice Builder in Jetpack CRM has a number of settings which are worth knowing about before you get started. You can find the key settings in two different settings pages in Jetpack CRM Settings:
Invoicing Settings
Load these settings by clicking Jetpack CRM Settings -> Invoicing:
These settings are invoice specific, e.g. reference (with auto-number) or Thank you message. You can also select whether or not to use tax, shipping, and discounts.

By default, the auto-generated reference is turned on, so your invoices will have a new reference when you create it. Also, you can add to the reference a prefix/suffix, and set the next number (will be auto-incremental) of the reference.
Business Info Settings
Load these settings by clicking Jetpack CRM Settings -> Business Info:
These settings allow you to set globally-used business information such as your business name or contact email.

Creating your first invoice
To create your first invoice you can click Invoices -> Add new from the top Jetpack CRM menu.
You can change any Invoicing information you want by editing it inline. Since v3.0
you can now also add tags:

Adding new lines to the invoice
You can add new rows to the invoice to have your invoices be the total of a number of line items. This is useful if you want to break down your invoices for your clients.

How to set the Invoice Status
The Invoice comes with a number of statuses. Draft (while you work on it). Unpaid, Paid and Overdue.
This controls what the client is shown in the client portal.

Extras
Custom fields
Since v3.0
you can now also use custom fields with Invoicing. Click here for the guide on custom fields.
Tax rates
Since v3.0
you can now also use custom tax rates with your invoices.