Installing the MetaMask extension on iOS is a bit more complicated than installing it on a desktop browser, but it is still possible. Here are the steps you'll need to follow:

1. Open the Safari browser on your iOS device and navigate to the MetaMask website (MetaMask Safari extension)
2. Tap the "Get MetaMask" button to open the App Store
3. Install the MetaMask app on your device
4. Once the app is installed, open it and create a new wallet or import an existing one
5. To use MetaMask as an extension in Safari, you'll need to enable the "MetaMask" option in the "Content Blockers" section of Safari's settings.
6. You should now be able to access MetaMask from within Safari and use it to interact with web3 enabled sites.

It's important to note that MetaMask for iOS is currently in beta, and as such, some features may not be available or may not work as expected. Additionally, it is always important to make sure you are using the official MetaMask app from the App Store and to be mindful of phishing attempts.