How do i create a .dwfx for Navisworks Freedom? (from Inventor .iam)

What can I do, to create a .dwfx, from a large Inventor assembly (.iam) -to be viewed in Navisworks Freedom?
I have Navisworks Simulate,
Should I just open my .iam directly in Navisworks Simulate, and save it as dwfx, or should I export my .iam to a .step and then import the .step? I think sometimes my STEP files become very large (like 200 MB) (though the .iam is only like 20 MB)
Do I need Navisworks Manage? .nwd file first?
Any tips? Maybe you know of any links to articles/forums/tutorials? If you have any experience with creating .dwfx files for Navisworks Freedom, please share that precious knowledege : )