Google Home’s Actions are available to use as soon as they're in the Services section of the Google Home app, no setup is required. 

For instance, to play the Akinator game, you never have to enable anything on Google Home. Just say, "OK Google, let me talk to Akinator," and the game will begin. 

It feels much more like vertical integration, like your Google Home is just getting smarter every day.

That said, some services, such as Uber or Todoist, do require you to link your account. To do this, open the Google Home app [on iOS or Android] then :-

Tap the hamburger button in the top left corner of the app.

Select More settings and tap Services. 

Scroll to find the Service you want to activate and tap on its name to load the details page for that service. 

Tap Link Account and follow the on-screen instructions to authorize your account.

To use any of the services available, you need to know the invocation for it. You can look up how to interact with a service by opening it’s page in the Services section of the Google Home app. 

Some examples of how to launch or interact with each particular Action will be shown at the bottom of its description page.

In many cases, as long as you know the name of the service, you can just ask Google to let you talk to it. 

There are a whole range of Apps that you can get these to work by saying a 3 part command in the following way ....

Part 1:    "Ok Google"        followed by

Part 2:   "Talk to"                   followed by

Part 3:   "xxxxxxxxxxxxx" where this is the name of the App / Service - here are some examples to try.

"Time Machine"

"Mister Grumbles"

"Misfortune Cookie"

"Quora" [where you pose questions]

"Today I learned"

"Domino's"

"Good Code tips"

and finally [for the non-sensitive only] the crudely named ...

"Unleash that fart"

N.B. When the Action runs it may walk you through how to use it with voice prompts as necessary