Set up their banks and credit cards to auto-import into our app. This will bring in at least a few weeks of data. Here's how

Import a file of their older transactions to fill in the earlier part of the year that wasn't captured by the auto-import. This is a one-time task, and you can even import expenses already categorized. Here's how

Import contacts via Gmail, Basecamp, or Highrise.

Manually create just open invoices and set up recurring invoices to be auto-created and even auto-delivered.