you have to go to "projects" and then you should see (having already researched colony cluster level 1) the option to cluster
when you cluster, it forms the 5 lowest clusterable colonies on your manage colonies screen
planets you cant cluster are:homeworld, U class
so make sure you have your biggest barrens(barrens have the potential to have the most land out of all the clusterable ones) are on the bottom, and make sure you have pooey planets above to act as borders to protect your cluster from attack
then cluster
the end