Create a countdown in C# (inside Coroutine)

I’m making a circular countdown timer, and I already have the sprites and the stuff ready to be used. My problem is just making the timer inside a coroutine (it will be started after a level finishes loading), so I need to count X seconds inside it and display them by reducing the countdown Image fill.
How do I make time count inside a coroutine and get how much time is left?
(using the update function is a no because the class I’m working at is a simple scene manager)

Similar to MacDX’s answer:

void DoStuff () {
    // Whatever you want to happen when the countdown finishes

IEnumerator Countdown (int seconds) {
    int counter = seconds;
    while (counter > 0) {
        yield return new WaitForSeconds (1);
    DoStuff ();

Maybe something like this

public Image countdownImage;

void Start()

private IEnumerator Countdown()
    float duration = 3f; // 3 seconds you can change this 
    //to whatever you want
    float normalizedTime = 0;
    while(normalizedTime <= 1f)
        countdownImage.fillAmount = normalizedTime;
        normalizedTime += Time.deltaTime / duration;
        yield return null;