Download and launch the app
Open the Google Play Store or Apple App Store, on your phone, and search for WhatsApp.
Its logo looks like this 
Tap install.
Open the app
Once you have installed it, either tap Open if you are still in the store or tap the WhatsApp icon which will now be on your home screen.
Review the Terms of Service
Read the Terms of Service and Privacy Policy, and then tap Agree and Continue to accept the terms.
Register
Select your country from the drop-down list to add your country code (+44), then enter your mobile phone number omitting the first 0.
Tap Done or Next.
Now tap OK to receive your 6-digit registration code via SMS (that’s your normal text messages) or by a phone call. To complete registration, enter your 6-digit code.
Set up your profile
In your new profile, enter your name, and then tap Next.
Allow access to contacts and photos
Contacts can be added to WhatsApp directly from your phone’s address book. This is useful as you begin to use WhatsApp more often (and I suspect you will!) So allow access.
Start a chat
Tap 
or 
then click on the person (or group) you want to contact.
Write your message in the text field.
Then, tap 
or 
to send
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