Example.cs

public class Example : SingletonBehaviour<Example>
{
    private void Awake()
    {
        Assign(this);
        
        // ...
    }
}

SingletonBehaviour.cs

using UnityEngine;

public abstract class SingletonBehaviour<T> : MonoBehaviour
{
    public static T Instance { get; private set; }

    protected void Assign(T instance)
    {
        if (Instance != null)
            Destroy(gameObject);
        else
        {
            Instance = instance;

            DontDestroyOnLoad(gameObject);
        }
    }
}