How Do I Flatten Photos In Photoshop?

Once you have finished your work on a photoshop file and you want to flatten the image, then click on "Layer" at the top of your photoshop screen, and at the bottom of this selection is the choice "flatten image."  Click on this to flatten your image.

I might suggest that you save your original work first under a different file name before you flatten the image, and then save the flattened image under a new name.  This will allow you the option to still make any changes to the original work, because you will be unable to do so with the flattened image that you save.