Before stepping further, lets first understand what "One page checkout" is? 

one-page checkout displays all elements of a standard checkout including basket contents, billing and shipping address, shipping options, and payment information on one page. Originally, one-page checkouts were put into practice in an effort to simplify the check out process with fewer clicks and fewer pages.

Now, let's move to how one-page checkout can be customized and used:

Step 1- To customize one page checkout, 

go to:

Campaign> One page checkout> Checkout form