Before creating an assembly, all associated Parts, Materials, Operations and Routes must be created
Navigate to Data⇒Production⇒Route
Select the part you want to be the main assembly and press EDIT.
When the part is open use the plus iconto add parts to the assembly.
When you click on the plus icon you will get the below popup window click on the sub assembly part tab.
Then click on the spy glass to search for the part you want to add.
Once you find the part you want to add double click on it.
The part you selected will appear against the part number, select the quantity that is required in the assembly of that part then click save.
The part will be added to the route click save or repeat the process if more parts are required in the assembly.