What is the process for roasting and preparing chicory for coffee?

As the page in @InstantCoffeeJedi's comment points out:

1. Mince the roots into equal particles with a knife. The roots are sturdy, so you’ll need a sharp knife, especially since the pieces shouldn’t be longer than 1 inch. If you don’t cut them in the same size, there’s a chance that they will not roast evenly.
2. Roast the chicory pieces. Arrange them on a shallow baking pan or a baking sheet and place them into the oven. Toast them at 350 degrees until their color changes to golden brown. If you can sense a coffee-like smell, this means they are finished.
3. Grind the roots. Place the roasted chicory roots into your burr grinder and grind them with the same coarseness as you grind your coffee beans. Ideally, it should be a fine grind if you’ll brew them in an espresso machine or a coarser grind if you have a French press, etc.
4. Mix chicory with ground coffee. The ratio is up to you. If you want the taste of coffee without the caffeine stimulation, you can try with a 4:1 ratio of chicory to coffee. Sometimes a 2:3 chicory-to-coffee blend of is good if you want to gradually reduce your caffeine intake. If you want to spread out your coffee supply over a longer period, adding four-fifths of chicory would be enough. Alternatively, you don’t have to add coffee at all for a true chicory coffee.
5. Grind the roots. Place the roasted chicory roots into your burr grinder and grind them wit.
6. Brew your coffee. Use the chicory and coffee powder mix to make the coffee as you regularly would. All the other factors such as time or technique should be the same.
7. Add some foamed milk, toppings, or flavorings to enhance the aroma and serve your chicory root coffee.

Basically you roast the chicory in the oven at 350°F (or 177°C) until they turn brown and after cooling them you mix 20-40% of their grounds with coffee grounds. Mixing chicory in doesn't really yield gourmet coffee so imprecise recipes are to be expected.