Bill me later is now known as Paypal credit, and is a paypal service that allows you to pay your purchase over time, hence "bill me later"..
You log into paypal and apply for a PayPal Credit account, and when u want to pay with bill me later, on checkout choose PayPal Credit as your payment method.
After that PayPal Credit will then be added to your PayPal account automatically.
About 10 to 14 days after your purchase ships from a retailer or your travel is ticketed, you'll receive an email notification that your billing statement is available online.
Upon that, just select payment options(preferences) and thats that.
Please note that the Annual Percentage Rate is 19.99%, and ofc there are late fees aswell, so i dont see why anyone would want to use this..
Most of these info's i sniped from
https://www.billmelater.com/help/index.xhtml , and there are many more informations regarding the service there, so i dont see why you are confused about the service,
it looks pretty much straight forward.
Is there any particular question you have?
cheers