What steps should be taken when using abstract classes in object-oriented programming?

When using abstract classes in object-oriented programming, the following steps must be taken:

1. Define an abstract class with the keyword abstract. Such a class can contain both implemented methods and abstract methods (without implementation).

2. Declare abstract methods within the abstract class — these are methods without a body that must be implemented in subclasses.

3. Create subclasses that inherit from the abstract class and implement all its abstract methods.

4. Use objects of subclasses to create instances, as the abstract class cannot be instantiated directly.

Example in PHP:

abstract class Animal {
    abstract public function makeSound();

    public function sleep() {
        echo "Sleeping...";
    }
}

class Dog extends Animal {
    public function makeSound() {
        echo "Bark!";
    }
}

$dog = new Dog();
$dog->makeSound(); // Outputs: Bark!
$dog->sleep();     // Outputs: Sleeping...

Thus, abstract classes define a common interface and partial implementation, ensuring polymorphism and code reuse.