In PathFinder, click the Parts Library tab.
Select a part you want to place in the assembly and drag and drop it into the assembly window.
Repeat step 3 for all the parts you want to assemble.
Choose Home tab→Assemble group→Assemble 
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On the Place Part command bar, click the assembly relationship you want to use.
Apply the assembly relationships you want between one of the parts you placed in the assembly and the other parts.
To position a different part, click the right mouse button.
Repeat step 6 and 7 until the parts are fully positioned.
Tip:
When assembling parts, you may get more predictable results when you apply the first relationship to a grounded or fully positioned part.
After you apply a relationship between two parts, the first part you select remains selected so you can apply additional relationships to the part.