In some mobiles this app came as an inbuilt app else you have to download it from the playstore.
Backing up is very easy, you have the option to backup manually or simply choose the auto mode and this can be done using either over your mobile data or wifi.
In your gallery select the photos you want to upload, then go to the share option where you have to choose google photos to upload it.
Don’t forget to sign up in to google photos using your email.
For auto upload go to its app setting and simply activate the auto upload.
You can also select folders like whatsapp image, fb image, bluetooth or downloaded images for auto upload, by default it will upload your camera folder images.
So this is a very usefull app to keep your memories safe.