Sub Tables are List Properties of Object Types. Whereas there are several other Help Resources explaining their use and their Data Source involvement, this Help Resource simply demonstrates how the Sub Table Object editor form is made.
The Sub Table Edtor Form is the equivalent of an Object Editor Form. Consequently you can create this form either by dragging / dropping appropriate controls onto a Form (as demonstrated in one of the other Help Resources) or by using a quick, one button method of generating the form.
The attached Video demonstrates how the "Create" button is used.