The software I use, PaintShopPro (PSP) doesn't have a duotone tool. So I read the course handbook carefully, consulted a couple of other text books and thought out the process from first principles using layers. All the settings were reached by successive adjustments. I hope I haven't missed the point entirely!

We already have have a warm-tone and a sepia image so, rather than launching into anything more ambitious, I decided to make a cool-tone image using a shade of blue as the underlay.

The cool-tone effect is deliberately exaggerated here and never would suit this image anyway.