Please choose and click the Donate Button for your currency:
You will be taken to the PayPal Checkout page where you can add your amount and click on the Make this a monthly donation check-box.
If that option is not available on the PayPal Checkout page (sometimes PayPal chose to make our lives difficult!) please go to this page instead.
Complete your transaction either using your PayPal a/c or credit card via PayPal.