Once in a project, fields are one of the ways Asana allows you to add more information to a project. They are filterable and customizable. To add a field, click "Add field".

You can either "Choose from library" which has field options created by Asana and APTA. 

To locate a pre-made field, either scroll through the options or search based on what you're looking for. If you find what're you're looking for click on the field to add it to your project. 

The field will add further to the right on your screen.

app.asana.com

This guide provides step-by-step instructions on how to customize fields in Asana. It explains how to add fields from the library, create custom fields, change the list view, delete fields, hide fields, and unhide fields.