Before stepping further, lets first understand what "One page checkout" is?
A 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,
Campaign> One page checkout> Checkout form