First, have a look at the explanation of importing data here (How do I import data into inFlow). When you`re ready:

1. Click Main Menu then choose > General > Import Data.
2. Select the Customer data type at the top of the screen.
3. Click the Browse button to select the file you’d like to import then click Next.
4. In this screen match the inFlow field name to the right column in your file by selecting it from the drop down list.
5. Once you’re satisfied with your matches, click Next.
6. inFlow will import your Customers and let you know what has been updated!