ClassWithCustomEditor.cs

/*
    Sample custom editor for a class in the same file.
    I've yet to encounter errors with compilation order.
*/

using UnityEngine;
#if UNITY_EDITOR
using UnityEditor;
#endif

namespace Project
{
    public class ClassWithCustomEditor : MonoBehaviour
    {
        [SerializeField]
        private Vector2 propertyInEditor;

        //...
    }
}

#if UNITY_EDITOR
namespace Project.Editors
{
    [CustomEditor(typeof(ClassWithCustomEditor))]
    public class ClassWithCustomEditorEditor : Editor
    {
        public override void OnInspectorGUI()
        {
            serializedObject.Update();

            EditorGUILayout.LabelField("Some customization here");

            EditorGUILayout.PropertyField(serializedObject.FindProperty("propertyInEditor"));

            serializedObject.ApplyModifiedProperties();
        }
    }
}
#endif