Put glue on edge of a part (blue line) and paste two parts together.
As well as this, put glue on edge of a part and add 3th part.
Got three parts together.
Add 4th part
Got four parts together
Add 5th part
When 5 parts got together, put glue on the edge of unit and paste a first part together.
Making one unit finished !!
It's needed 12 unit.