To get started with creating Custom Fields for Leads, click on Settings from the top menu.
Select Custom Fields from the left menu.
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.
A new custom field will reflect in all your new leads created henceforth and also can be viewed here. You can edit/delete fields from here.
The Custom Field(s) will be visible inside your new leads created henceforth. Just click on the newly created lead in your pipeline and select Fields from the left side of the menu. The Custom Field will be visible and you can add it's respective value beside it. You can add/delete a field pertaining to this lead. You can delete any pre-existing field and add a new field more suitable for this lead.