When working on a parent form in the forms designer you can create the a new associate items field simply by selecting the following options

Once you click on save the process will generate a new list with the name you provided and the only fields on this new list will be the Title. You will need to add your other fields for the child and the view when designing the form and child lsit view. 

Above is the easiest way to create new associate items. However, it is still valid to build your child lists first and design the form for it and then add the associate items field to the parent form.

To learn what the use cases are for using an associate items using the infowise associate item content type click here.