Head to Admin > Meals > Guest Meals
Then Add New Guest Meal:
Here you can enter details of the meal selected, the number of meals required, descriptive notes about why the meal is required, and whether it is to be charged.
If the guest is paying for their meal, select Charge Guests for Meal. When they come to pay, go to Guest Meals and click Filter this Page by Dates. A Pay Button will appear enabling you to enter the payment: