Fill in as much information as you have.
If most information and attachments are already in an email from the customer, you can wait for the pending project in Basecamp to be created and add them at that point with the:
"Email content to this project..."
link at the bottom of every project page.
Fill out any specific notes related to the delivery of items and due dates in this project.
Make special reference if items have different delivery milestones separate from the overall project due date.