If you would like to quickly and easily create a collage of up to 6 photos, try Google Photos app. Once you select the images you will see a collage created. If you tap on an individual photo you will then see the edit and replace options where you can enhance, crop, as well as adjust brightness and add filters. It is also nice to know that if do tap on an individual photo in the collage you can move the image up and down as well as left and right.
One last item to point out is that you have options available to you at the bottom of the screen. These options allow you to add some artistic options.
It is important to note that if you would like up to 9 images in your collage use the web version of Google Photos.
Embedded below is a 1 minute and 44 second YouTube tutorial.