First you need to follow the instructions found here to set up your form:
Notice that you can let the form appear for all users, a specific group or for batch or single-file uploads. Once the form is set up, it can then be called into play during a Web-based file upload. Basically, the form will automatically pop-up during a Web-based upload for which the conditions (e.g. group, batch, single-file upload) are met.
In most cases, you would like to do something to the information entered into the form. For example, you might want to include it in a notification email or perhaps append it into a file. In this case, you would need to create a trigger that uses the File Upload event type.
If you have more than one form, then you can tell the trigger to act only on a specific form by using the UploadFormName variable in the trigger conditions.
For example, if you have forms named Form1, Form2, Form3, and you want a trigger to act only on an upload that uses Form1, then you can add this to your File Upload trigger condition:
e.g. UploadFormName = "Form1"
Once you get to the Trigger Actions during trigger creation, you'll then notice that, if you click the Variables button, the list of variables will include the fields you included when you created the form. You can then use those variables to enter information gathered by the form into an email, text file, etc.