Why do you need this? In my opinion it is not the best idea to spy in such way. If you think she is doing something bad behind your back, firstly ask her about it. If you can't talk to your partner, your relationships are bad. But still if you want to spy on WhatsApp, here
spy-apps-software.com/best-whatsapp-spy is good application for you. All is quite simple, I think you will understand how to use it.