All you need is a small light in the coop. Chickens don't like to go into dark places and if you have a light in the coop, they'll be drawn to it as it begins to get dark outside. Turn the light off when you lock them up.
Do that for about a week and they'll learn to go into the coop as it begins to get dark outside.
Do that for about a week and they'll learn to go into the coop as it begins to get dark outside.