First, login to Zoho CRM. Then, navigate to the module of your choice by clicking on its label at the top of the page, in this example: Leads.

Look at the URL in your address bar and note the number directly following /custom-view/ in this example 299641200007501

This number will change any time you click on a different view using the panel at left (Public Views or Custom Views). This is your Custom View ID.