Once you add and item to card and start the checkout process you will reach this page below (may look slightly different on mobile devices)

checkout page oceaneva
At this point you can either scroll down and enter your credit card info and complete the purchase in full or you can click "shop pay" if you wish to pay in installments.
You then enter your email to continue(if you have already this part will be skipped)
Shoppay Enter Email
You will then be asked to enter your mobile #.

You may then be sent a code by text to enter.

After you enter all requested information such as you shipping address you will get the screen below.

Choose monthly installments and enter your credit card information and click next. 
Don't worry you card will not be charged yet.  You will pick you plan on the next page.
    Monthly Installments Oceaneva
    Afte this follow the prompts.  It may ask you for verification information then it will let you pick your payment plan.