Tap Clients > + Create. You can either:
Import from your phone contacts and pull their details in automatically.
Enter manually and fill in the details yourself.
Standard fields:
Name
Phone
Company name
Address is powered by Google search, or you can enter one manually. Toggle on Service address is different if the address where work gets done isn't the same as the billing address.
Advanced fields (tap Show advanced fields):
2nd and 3rd email addresses
2nd phone number
Fax number
ID or Tax #
Tags
Rating (up to 5 stars)
Notes
Tags and ratings are for your own reference, a way to label or score clients however makes sense for your business.
If you have an existing list of clients, you can import them all at once from a CSV or Excel file instead of adding them one by one, see Importing your clients in bulk.