How do I respond from my watch to whatsapp?

You don't. You can only receive notifications from WhatsApp on the Apple Watch. WhatsApp does not have an app that supports the Apple Watch at this time. You will have to wait for them to provide one. You will have to answer from your iPhone.

Begin by installing the WhatsApp messaging platform on your Apple iPhone.

Next go to your iPhone Settings and select Notifications then WhatsApp Messenger.

Then activate the Allow Notifications and Show in Notification Centre option.

Open the Watch app from your Apple iPhone, then open Notifications in the Watch app.

From here you can view all Apple Watch-supported apps, so scroll down until you see the WhatsApp Messenger app then activate the notification button.

You can now receive every WhatsApp notifications on your Apple Watch

Some WhatsApp features cannot be supported due to the Apple Watch’s space restriction and tiny screen.

This method essentially configures your Apple Watch to mirror the app's notification alerts from your iPhone.

Or you can use a 3rd party app called chatify from the iphone app store.