-Choose the image you want to change into Sepia. I will be using this one:

-Go to Image> Adjustments> Desaturate (keyboard code Shift+ctrl+U). If your image is already grey go to Image> Mode and make sure it is set to "RGB"

- Go to Layer > New adjustment layer > Levels. Click Ok and then select "RGB" from the drop-down list and fill these values in:
Input: 20 1,10 235
Output: 0 255

-Go to Layer > New adjustment layer > Selective colour. CLick ok and then check "absolute".
Select "Neutrals" from the drop down menu and fill these values in: -15 -1 +17 -7 DON't hit ok yet!

Select "Blacks" from the drop down menu and fill these values in:: +16, 0, -1, +100 (if your image is light) or -1 (if your image is Dark with intense shadows)

-Tada! Your image is ready!