To add a Reservation to a Sale, either Create a new Sale (see How do I create a new Sale?)  or Edit a Sale (see How do I edit a Sale?)

Reservations can also be added to the Sales in the Checkout screen, see How do I checkout a Reservation?)

Once you are on the Edit Sale screen, you can EITHER:

Select a Reservation from the CUSTOMER's UNPAID RESERVATIONS scrolling menu, or a Service from the SERVICES scrolling menu (this is a stand alone service, that is not attached to a particular reservation)

OR

Tap + Add Reservation/Service

Tapping + Add Reservation/Service will display a list of this customer's existing Reservations. Tap on the one you wish to add:

You may also tap Add Manually to display a list of Services which may be added to the Sale. These are stand alone Services, which are not included with a Reservation:

The added Reservation/Service will appear in the SERVICES section:

Proceed with adding any required items and payments to the Sale (see How do I add a Payment to a Sale?)

Then close the Sale