You are not required to add customers into the database prior to creating a document unless the customer is to hold an account with you
If you want to add customers you can do so by clicking either Add New Customer or Add Account Customer buttons

When creating the customer, you will be asked for some basic information.
 

The account number field does not have to be filled in, if it is left blank it will automatically generate an account number based from the surname and next available 3 digit number, such as SMI001 for Mr Smith, however if you tick the 'Is this a business name' field lets say for SWS Solutions, the account number would generate from the first word and would become SWS001

Standard Customer Creation Screen

You can also specify a custom labour rate, this will be looked up when adding labour items to a document as long as the customer is selected first.

The Regular customer option, just adds an extra ability to locate the customers easier when browsing the database