How to add your own form to the checkout process?

Hello!
Could you please tell me how to implement this? I need to remove the shipping information from the checkout process and replace it with a small form of my own. Removing it is clear, it's in the checkout settings. But how do I add it? I need a form that includes first name, last name, phone number, email with confirmation, and possibly something else.
I'm using Drupal 7.22 and Drupal Commerce. I tried to do this with Commerce Extra Pane by adding a webform, but it gets submitted separately. I need it to be submitted, return to the page, and confirm the purchase. It doesn't feel right.
Thank you in advance for your response!