Summarize the following:
To get the most out of Google Photos features, such as automatic (and unlimited!) photo backups, install the app from your device's app store. If you see an UPDATE button, Google Photos is installed but out of date. Tap UPDATE to get the latest version of the app. This makes it so your photos automatically upload to Google Photos. Otherwise, your phone will automatically back up your photos when you're not on Wi-Fi. This can be costly! " High quality: This will work great for almost everyone. You'll have unlimited space for uploads. Photos and videos will upload at a maximum of 1080p full HD resolution and 16 megapixels. Original: You'll be able to upload higher quality photos and videos, but they'll count against your Google quota (no matter their size). Choose this option if you're a professional photographer who already pays for extra space in the Google cloud. " Now you'll see a brief tutorial. When it's complete, you'll arrive at the Photos screen.

Summary:
Open the App Store (iOS) or Play Store (Android). Tap the Search box. Type google photos. Select "Google Photos" from the search results. Tap GET (iOS) or INSTALL (Android). Tap OPEN. Tap GET STARTED. Sign in to your Google account as prompted. Make sure "Back Up & Sync" is on. Make sure "Use cellular data to back up" is off. Tap "Continue. Select a photo upload size. Tap "Continue. Swipe left through the tutorial.