First and foremost, points bought on Steam can only be used for an RO1 Steam account and vice versa. Please don't use the WarpPortal site to buy your points as these will not be applied to your Steam Account. If your Kafra Cash button doesn't work, check out this article
to get it working!
To begin purchasing your Kafra Cash, click on the adorable Cash Shop button in the top right of your screen! It will be just to the left of your mini-map!
You will then be brought to the Kafra Shop window, and from there you'll click on Buy VIP & Points.
It will bring you to Item Mall Point page. Please select your desired point value and click on Checkout. To confirm which account you're purchasing for, highlight the URL Address Bar and search for "accountname=". The value for accountname= will be the account you are purchasing for!
From there, you'll be taken to a Payment Method page. If you don't have any credit in your Steam Wallet, you'll also be asked to Add Funds.
Now you're at the home stretch! Just add the funds you need, and you'll get your Kafra Points to spend on all the fun Kafra Shop items you want!
If you run into any trouble with your payment, submit a ticket to WarpPortal Support