First, setup AdMob and add to your app settings as described here.
Create Your Interstitial Ad Units
Next, create an interstitial ad. You will need to create a separate intersitial for iOS and for Android.
Go to the Admob app you created for iOS, click Ad units on the left, and click Add ad unit (as shown below).
Select Interstitial, give it a name, and click create.
Copy the ad unit ID (the second string), that's your ad unit ID.
Repeat this process for Android (if you want both ads on both platforms). Go back to your apps, choose the app you created for Android, and then create another interstitial ad unit.
In the sample code you need to replace the app unit IDs with the ones you created above.
When you are finished, save the file as a .js file, and upload to your app under the Settings tab as described here.