Introduction

There are two methods, a simple one and a more advanced one.

Simple Method

• Filter -> Decoration -> Old Photo
• Defocus -> Off
  Sepia -> On
  Work on Copy -> On

Advanced Method

• Convert the Image to gray scale:
  • Either with Image -> Mode -> Grayscale
  • or with Color -> Component -> Channelmixer
  • Eventually tweak the result with Color -> Brightness/Contrast
• Create a new layer “Sepia Mask”
• Set the foreground color to RED=162, GREEN=138 and BLUE=101* Fill the new layer with this color.
• Add a Layer Mask with White (Full Opacity) to the new layer
• Select the image layer, select everything (Ctrl-A) and copy it (Ctrl-C)
• Select the mask in the Sepia Mask layer and paste the selection (Ctrl-V)
• Link it to the mask by clicking the Anchor butten on the button panel
  below
• Invert the colors in the Mask layer: Color -> Invert
• Change the Mode of the Mask layer from Normal to Color in the drop down on top

Source: Sepia Toning