Once you have found a stock photo of a film border, open it in Photoshop along with the original image. Place the original image over the film border and resize it so the image only appears in the center of the film border.
Next, double click on the film border to convert it to a regular layer and bring it to the top of the layer stack.
Change the blending mode of the film border layer to ‘Multiply’ to set the effect. MultiplyBlend Mode will make the dark areas of a layer visible and the Light areas invisible, allowing the original photo to show through the center.