1Make sure you set up emails
The default configuration uses php's built in mail() function. This works fine on some hosts, but many block this funtion, which means the Duet won't be able to send any email. Email's are a critical component of Duet's functionality so it's highly recommended that you turn on SMTP for outgoing emails.
Most server's have built in SMTP capabilities. You can get the correct settings from Cpanel or directly from your host.Email Setup Instructions
2Setup Payments if you plan to send invoices
Invoices work out of the box, but if you want your clients to be able to pay you, you will need to setup payments.
Don't forget to turn off test mode when you're ready to start accepting real money! :)Payments Setup Instructions
3Do you want to reply to messages directly from email?
If so, than you will need to set up incoming email. This is 100% separate from the email setup you did above, and is slightly more complicated to setup. The easiest path is to create a free gmail address that will only be used for DuetIncoming Email Setup Instructions
4Recurring Invoicing and replying to messages from your email require more setup
If you're using recurring invoicing or would like to reply to messages directly from your email, you will need to set up scheduled tasksScheduled Tasks Setup Instructions