Sepia tone is a reddish-brown monochrome tint that gives a warm, antique look to a photograph. In the early days of photography, photographs were developed with sepia, derived from cuttlefish ink, in the emulsion.
How to Create a Photoshop Sepia Effect Action
Open the image in Photoshop.
If the image is in color, go to Image > Adjustments > Desaturate.
If the image is in grayscale, go to Image > Mode > RGB Color.