This is actually a straightforward procedure. The first thing you do is find it in the app store. This is different from Google Play, because Apple would prefer you keep everything on Apple products and stores. However, once you search it up in the app store, it should be the first app that pops up. From there, you go to the Whatsapp page, and click download. It’s a free app, so there is no hassle in putting in a credit card, debit card, or gift card number or authentication.
From there, you can use Whatsapp as if it was any other app downloaded from the app store. It is really that simple to do, regardless of the phone. The only thing to be weary of is that the older models may not support it. If that’s so, the phone will typically tell you.