You will need the following things before you can successfully use orderthisprint.
A PayPal account is an easy way for you to accept payments online. PayPal accounts are referenced by email address. For more info on setting up an account visit PayPal's website.
If your art sales page is hosted somewhere else instead of using the PayPal shopping cart the order link will direct them to that site. For example if you are on Etsy (which has a good mobile site) you will put the link to the order (on Etsy) page of that print in this field.