It's very simple, there are two possible parameters currently, size (which is optional) and data (which is mandatory)!
Please Note: The requested size must be no more than 1000x1000 pixels
Example Request: http://qr-api.ashleyprimo.com/?size=250&data=This%20is%20an%20example
This above example, will generate a QR code with the text "This is an example", with a size of 250x250 pixels- it's that easy.