First off you want to decide which identifier you wish to use to choose. The most used one is email but you can also use the customers ID or phone number for example.
Once you know you can start by creating a new campaign (see the guide for that here).
And once the campaign is done you can click on the "Edit audience" button to choose your recipient :
Here you can define the identifier you wish to use in the search bar and click on it :
Then start typing in the email or ID that you wish to use and Custobar will start showing the recipients that you can use. Choose one by clicking on it.
After that you are ready to send it to your specific customer by clicking on the "Save action audience" button :