How can I upload a customized form onto a blank jotform?

Please follow the steps from the following guide to clone your own form:

https://www.jotform.com/help/27-How-to-clone-an-existing-form-from-your-account

The cloned form will be automatically loaded in the form builder. You can update the form before embedding it to your website. 

Let us know if you have any questions or if you need further assistance.