To enter a 12 constellations instance, click your Space Station, then click Instances. Click Constellations and click the sign which is illuminated in blue.
Then, click any of the illuminated planets and set it up like every other instance.
Like Scenario Instances, this will not result in permanent destruction of warships and only He3 will be consumed.
The following link is an interactive view of all of the Constellation Instances, ships and information. A very useful tool.
The 12 ConstellationsEdit
The 12 constellations are:
Choose an instance: