On an Android phone or tablet, open the Google Photos app and sign in to your account. At the top right, tap your profile photo or initial. Select Settings/Back up & sync from the dropdown menu that appears next to it and tap Back up & sync. You will then have a few options on how you want to back-up photos: for just yourself only (this is default), just full resolution backup, off, backing up whenever Wi-Fi is available, all backed-up photos automatically uploading if there's a network connection available etc. Tap "back-Up when wi-fi" checkbox