custom field at answer form / step by step guide

hi, i’m sorry cause i repeat the same question but i want to know how can i add custom field at answer form (a really step by step guide if is possible).
I read the orders at this link https://anspress.io/resources/faq/anspress-form-and-validation-api/ but i can’t understand where (in which file) to put a code, how to put a code etc.
If you know and you have time please submit a guide about custom fields………thanks for the support.