create admob account
click monetize new app
search for your app if you have already published the app otherwise add it manually.
fill out the form as prompted
at some point you will see an ad unit id which looks like: ca-app-pub-12345678998764/132165498
each id will correspond to the specific ad you make, you can have multiple ads in a project.
interstitial ads are full screen ads.
in construct 2:
add an admob plugin
copy/paste the 'ad unit id' to the appropriate spot in the properties panel.
control when they appear using the event sheets
you need to preload the ads before calling on the system to play(show) the ad. think of preloading like cocking a gun before firing.
leave test mode on 'true' to get a dummy ad that will act like the real one, switch to 'false' when you are ready to publish.
I normally leave overlap on 'yes' i have had weird issues when it's on 'no.'