this is probably the easiest way
If your customer already has an account - you can "Act On Behalf" of customer
In your Admin panel select Customers - find the customer name and the first icon is a little man - it is the Act on Behalf, if you click on it it logs you the Admin is as the customer and you can proceed to order, pay, etc. If you are using Stripe in this manner you should have Stripe enabled on your site.
If you customer does not have an account you can add the customer in the same Admin panel, Customer - click on the Plus (+) by the select group and fill in the customer information, then proceed with the act on half instructions above.