For the prerequisite steps, such as how to install Unity Advertisement package and integrated into project, please have a look at this:
Unity: Integrate the Advertisement package and monetize your game (Part 1/4)
Install the Advertisement package and setup environmentThere are few things needed before integration, and first of all you need toinstall the Unity Advertisement package: Step 1: Install Unity Advertisement package:Till now, the latest version is 3.4.7, you can use newer version if you like. …

Here is a simple and easy use case of how to put rewardedvideo ads in your game. Image we have a button in the game named extra life. The players will gain an extra life (extra coin, gold or anything you want) when they finish watching the rewaredVideo:

For this extra life button, we create a script attached to it (This script is from Unity doc):
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Advertisements;
using UnityEngine.UI;
public class RewardedVideoButton : MonoBehaviour, IUnityAdsListener {
#if UNITY_IOS
private string gameId = "3685328";
#elif UNITY_ANDROID
private string gameId = "3685329";
#endif
Button myButton;
public string myPlacementId = "rewardedVideo";
void Start () {
myButton = GetComponent <Button> ();
// Set interactivity to be dependent on the Placement’s status:
myButton.interactable = Advertisement.IsReady (myPlacementId);
// Map the ShowRewardedVideo function to the button’s click listener:
if (myButton) myButton.onClick.AddListener (ShowRewardedVideo);
// Initialize the Ads listener and service:
Advertisement.AddListener (this);
Advertisement.Initialize (gameId, true);
}
// Implement a function for showing a rewarded video ad:
void ShowRewardedVideo () {
Advertisement.Show (myPlacementId);
}
// Implement IUnityAdsListener interface methods:
public void OnUnityAdsReady (string placementId) {
// If the ready Placement is rewarded, activate the button:
if (placementId == myPlacementId) {
myButton.interactable = true;
}
}
public void OnUnityAdsDidFinish (string placementId, ShowResult showResult) {
// Define conditional logic for each ad completion status:
if (showResult == ShowResult.Finished) {
// Reward the user for watching the ad to completion.
Debug.Log("Player gain extra life");
} else if (showResult == ShowResult.Skipped) {
// Do not reward the user for skipping the ad.
} else if (showResult == ShowResult.Failed) {
Debug.LogWarning ("The ad did not finish due to an error.");
}
}
public void OnUnityAdsDidError (string message) {
// Log the error.
}
public void OnUnityAdsDidStart (string placementId) {
// Optional actions to take when the end-users triggers an ad.
}
}
When user click the Extra life button, a test adv will be displayed.
