In Google analytics create a new property or go to an existing one and get the tracking ID. It has the following form: UA-XXXXXXXX-X (where X are numbers).
Go to the admin section of your alf.io instance, insert the tracking id and save.
In the admin section of google analytics, after selecting the account and property, in the right there is a "Goals" link as depicted below. Click it.
There is a list of goals and the possibility to create a new one. Click the "+New Goal" button:
Get the short name of your event. For example, if your alf.io page for selecting the tickets is at https://example.com/event/my-event/, the short name will be "my-event".
Then in the first step, as pictured in the image below:
Give a name to the goal and select the Type as "Destination". Click "Continue".
Now, the final configuration:
/event/my-event/reservation/[^/]+/successas a value
Click save and enjoy!