If you have only a small number of contacts to add, you can do it manually. If you have a large number of contacts, there are other methods to do it more efficiently. See the article covering all import methods.
To add or edit a contact manually, go to the contact list:
Then, fill-in manually the info for this contact:
For more info, see the article on project field management
Editing a field manually is less risky if one of the fields's value to edit is the primary key. Indeed, as it is the primary key that is used to determine if a contact must be added or modified during an import, if the key has been changed, the contact will not be overwritten on import. A common example is a client that uses a new email. Importing this client with a new email will create a second profile for that client. To avoid duplicates, you can, when possible, edit the contact's email manually. Make sure to change this info in your import file as well.