To create new estimate go to Sales -> Estimates -> Create New Estimate or by clicking on the Build icon on the top left side and click Create Estimate
Client – Before selecting a client you must have added clients Customers->New Customer.
When selecting a Customer the customer informations will be fetched directly from the customer profile you created. Another option is that you can click on the edit icon and directly add the desired customer informations, this also is applied for the billing informations they are also fetched when selecting a customer.
If you want the billing details to be auto populated when selecting the customer you need previously to have added Billing details in the customer profile.