Start with one simple picture frame. This one is from a package of pressed paper and cardboard frames I got in my local craft department.

Remove plastic/glass that is used to protect photos. (You don't want to get anything on it.)

I gently separated the front from the back to make it easier. (You can re-glue it later)

Gather your images.

Attach images with even layers of Mod Podge (or other decoupage medium), leave a layer or two over the top to seal everything in. Let dry.

Using a hot glue gun (or other prefered glue)attach the front to the back, leaving the space for the pictures open. (In this case that was the top.)

Find a picture for the inside and enjoy.