Easy. You don't really have to do anything. As soon as your Viking Clan becomes active, your free extra bundle of data becomes active and you can use it. In order to get the most out of your data, you always use your oldest bundle first (the one that is activated first, so that could also be your normal subscription bundle). 

The validity and timing of a Clan bundle is independent of your subscription month. So your Clan bundle always has its own start and expiry date. If you welcome a new member to your Viking Clan, or if someone leaves the Clan, this will also be the new start date of your new Clan bundle (with more or less GB). Just like the other Viking advantage with which you call all other Mobile Vikings for free, the Clan bundle mobile data can only be used within Belgium.