Setting up payment through PayPal is really easy! Just head over to PayPal.com and sign up for an account. Once your account is set up and verified, login to your MiiDuu admin page and navigate to the Payments page (Setting -> Payments). Select PayPal and click Edit. All you need to do is enter your e-mail! There are more customization options, however, such as setting "Transaction Method" to "Sale" and "Order Status" to "Processing", but these are entirely up to you.
Make sure you select "Enabled" before clicking Save so that your customers can select this as an option!