how do you convert greyscale images to sepia?

The Actions tab has a action to create sepia images, click and done.

If i'm not mistaken you can also convert the greyscale image to RGB and then access the Photo Filters- located in the Adjustments menu. Photo Filters only work on RGB images so you have to convert the greyscale image to RGB before using it. It can also be applied as an adjustment layer, which gives you much more control and precision than the Actions tab.

Here's a method I use...

Convert your greyscale to RGB

Hold down CMD + OPTION + ~ to load a luminosity selection

Invert the selection

Choose Image>Adjustments>Variations and check the Midtones button then set the slider to the second or third notch from the left. Click once on More Yellow and once on More Red.

Use Hue/Sat to adjust colour if needed.

Chip

anything wrong with duotone?