To invoice a casual or walk-in customer, select the red + Create new... button and select Walk-in Retail sale.

This will bring up the Walk-in sale screen. Select the required product or service using the search field.

You can add a discount or surcharge at this point) if required (click here for guide). 

Select the Take payment button.

Enter in the payment method(s). Select Print a receipt checkbox if required.

Click on the Complete payment button.