Here’s how to create a collage using Google Photos for Android.
  1. Launch the Google Photos app on your phone or tablet. Tap on the “+” on the top-right of the screen, and then select Collage.
2. Select from 2-9 photos that you want in your collage. Then click Create.

3. Your collage will auto-assemble and can be found by clicking Albums at the bottom right of your screen
It’s a simple tool that Google gives to its users. Give it a try. You might enjoy it.