Step 1: Create your elements on Build Form tab of Contact Form
On Form tab you can delete the default form elements and add yours. To do it, you need to click on Delete icon, click Yes to confirm you're sure that you want to delete it, and then when you removed all of them you need to click Add to fill the form with the content you need.
Form Element consists of several sections. Label section is the name of your Form element and also it refers to Field ID that you can see at the bottom of menu, if you change label, field id will be changed consequently. Field IDs will help you to change the information on Mail tab. Placeholder will show an example of filling this field. With the help of the next option you can enable/disable Make this field required. You also can add Description to the field. In Column Layout you can choose the number of fields columns to display.
Here is an article describing Form elements in details - Types of elements available with Contact Form.
When you have added the elements you need, you can change Header and Footer of your form.
In Header section you can choose if you need to display the header or not, change Header Title, Header Text below the title and choose Header Picture if needed.
In Footer section you will find only one option - Footer Text where you can add the message below your form if you'd like to.