Collage is a way to assemble many photos into a single photo. This is a better way to remember memories within a single photo. However, There are many applications to create collage’s. I think Picasa, One of the Google product’s, is the best application out of all to create collage’s.
Here is the step-by-step procedure of creating collage.
          1. Open Picasa and select “Picture collage” from “create” tab.

          2. After selecting, You can see the window opened as shown in the below screenshot.

          3. In this window, select “clips” tab. There you can see all the images. Now, Just drag and drop the images (which you are going to create collage with) into the right pane.

          4. After draging the images into the panel, Just edit them as you want and save the collage.
          5. Now, you can see the collage saved in the specified folder.