They would be arrested upon setting foot in a country that is part of the ICC. Similar to how interpol issues arrest warrants. However, since these are high ranking officials that will just not travel to these destinations it's a pointless exercise. It does however hinder these people from going on speaking tour after they have left office which many do.

How exactly this would play out if a country tries to arrest a head of state which travels with security is unclear. I highly doubt any leader or nation would attempt to travel to a country where there is that risk of such a confrontation.