Thanks for reaching out to us!
Start by opening up the background image, then drag on the foreground image from the Photo Basket at the bottom of the editor. Click the foreground image, and a Photo Properties dialog will appear with an Erase icon. Click that Erase icon and paint around the person to delete the background. We have a brief demo video of this here: https://www.youtube.com/watch?v=ciYtYIWIXOs
If you have any other questions, please let us know!
– Regards, The Ribbet Team