It sounds like you're struggling to position the components/bodies relative to one another. Is that correct?
If the Move command is not sufficient to accurately place the body or component you are looking to combine, I would suggest using the Assemble > Joint command. Both bodies need to be converted to components first to be able to use this command. To do this you may right click on a body in Browser and select the Create Components from Bodies command (if you haven't already done this).
You can use the Edit Joint Limits values and offset values to precisely position the component relative to one another. It may help to create geometry in the component bodies that will make it easy to apply assembly joints to the components (e.g. holes, profile cuts, etc..). You can also create Joint Origins in place of missing geometry to help position your components. Joint origins can be used to create a "snap" point when there is no existing geometry.
More information on these topics can be found in the help files, below.
http://help.autodesk.com/cloudhelp/ENU/Fusion-Function/files/GUID-5AE0AFB3-47D1-42D9-AB63-FC9A94502F...
I hope that helps, please let me know if you have any questions.
If you share the design here, I will be happy to create a video containing some suggestions. You can create a public link of the design and post it in a reply to share the design.
How to Share a Public Link:
http://knowledge.autodesk.com/support/fusion-360/getting-started/caas/screencast/Main/Details/2b2054...
Thanks,
Nathan Chandler
Principal Specialist