To search for an existing customer click on the 'Search' sub menu under the main Customer menu. You will then be navigated to a search page as shown in the image below:
Customer Search Page
To start searching simply enter in the search criteria into the relevant field and click on the 
button.
After the search is completed a results grid will display on the page. For example searching for customers with the text 'tms' in the Customer Name field produces the following results:
Customer Search Results
To select the customer record you are interested in, simply hover over the row you wish to select and click once. This will then navigate you to the customer record in an edit mode (see Amend a Customer help page)
If no results are found in the search the following results screen is displayed:
No results found screen
Exporting Results to Excel
You will also see an 
button, which when clicked will launch a file download prompt screen where you can choose to either open or save the Excel file produced by clicking the button:
File Download Prompt