When you have installed, you need to set it up and enable it.
Go to Extensions --> Plugins, search for Fields and publish it.
There are a no fields to setup in the plugin itself, only a Download ID which you need to update the PRO version.
Go to the component you want to use custom fields (in this example i use the Joomla articles)
First create a Field group
The name you give to the Field group will be shown in the article as a tab and the description will be shown as information.
Now we can create our field
Go to Fields and create new one
Give the field a title and select the Custom Fields type note: after saving the field you can't change the type.
You can enter a description, select required and enter a default value if you like.
On the second tab (options) you can setup:
Placeholder: Placeholder text which will appear inside the field as a hint to the user for the required input.
Render Class: The class attributes of the field when the field is rendered. If multiple classes are needed, list them with spaces.
Edit Class: The class attributes of the field in the edit form. If multiple classes are needed, list them with spaces.
Show Label: Show or hide the label of the custom field
Show on: Site or Administrator or Both
Automatic Display: Joomla offers some content events which are triggered during the content creation process. This is the place to define how the custom fields should be integrated into content. After title, Before content Display, After content Display or None (choose none when you would like to render the custom field manually)
After this steps you can save the custom field.