To get started with creating Custom Fields for Organization, click on Settings from the top menu.

Select Custom Fields from the left menu.

Scroll down to find Custom Fields for Organizations.Under the label Custom Fields for Organizations, you can add new custom fields to all your new organizations by clicking on the blue button saying "+Add Field". 

Once you click the button, a pop-up window opens up saying Add New Field. Type the field name in the text box beside Field Name and then click on Add Field.

The new custom field will show up in all your new organizations created henceforth. You can edit/delete fields from here.

To see your Custom Fields in an organization, just go to More-> Contacts-> Organizations -> Create New Organization and you will be able to see an option of Custom Fields where your fields will be visible.