How do I combine multiple mockups into a single image?

1. Make the first image in a larger size, move the mockup to the left.
2. Frame your favorite photo into a mockup.
3. Tap "Set Output to User Image" in Background to load the output image as the background.
4. Merge the next mockup to the right.
   
   *The background can be transparent, but it's easier to control if you specify it.
5. If you want, you can crop the image in "Photos".