An Assembly is a file type that allows you to place multiple parts into a single file and constrain them together to create a system.
 
 
Press the ‘+’ button and select ‘Create Assembly’
You have created a new assembly
 
 
Press insert button
Select a part or assembly to insert into the assembly
Press the checkmark and confirm the assembly
 
 
Click Fastened to start the mate
Select the first surface, point, or edge you would like to fastened
Select the second surface, point, or edge you would like to fastened
In some cases you will need to flip the primary axis to the correct orientation
Press the check mark and your fastened is complete
Make sure there aren’t any errors such as over constraining
 
 
Right Click on the part you would like to ground and press ‘Fix’ and that’s it!