First, navigate to a customer's profile (Customers >> Customer List >> Select desired customer name) and then to their membership tab where you will see the option to Add Membership.
On the next screen, you'll be asked to select a membership plan to add to your customer.
On the next screen, you'll be able to adjust the start and expiration date, add a discount code, amend the pricing of the membership and decide on the payment method. If you are using an online payment processor with TeamUp (e.g. GoCardless /Stripe) then you will also be able to enter their card details to take payment.
Finally, select Purchase.