Firstly clustering is the like making 5 normal planets into a bigger planet
Cluster 1 (C1) =5 planets
Cluster 2 (C2) =25 planets (Or equivalent of 5 C1s)
Cluster 3 (C3) =125 planets (Or equivalent of 5 C2s)
So before you want to use the project you must have at least 5 free normal basic planets (e.g. Barren, oceanic, desert, balance..)
Dead Class, U Class, Similare or infected cannot be cluster unless they can become basic planets.
Then all you have to do is .
Click "Project"
Click "Colony Upgrade"
Click "Colony Cluster Lvl 1"
Click "Confirm"
Now you should have your Cluster 1