To install the Kardia Watch app, you start by opening the app named "Watch" on your phone. Scroll down and you will arrive to a list of applications. You should be able to find "Kardia" in that list and tap the install button next to it. Full Instructions for install here.
If Kardia is not present in that list, its an indication that your watch has not been updated to watchOS 4 which is required.
To update to the needed watch OS:
- Open the "Watch" app on your phone
- Go to General > Software update.
- You will be presented with any software updates available to you.
- Install any updates available
- After completing the update to watchOS 4, the Kardia app should now be available to install to your watch according to the watch app install instructions.