Right now when you click on the Sources Tab in FamilySearch Person Tools the left pane shows you the sources in RM and the right pane shows those in FS and you can copy from one to the other. That's great. The Sources shown are organized/sorted by Source Title then Event that it is applied to. One thing that makes it hard to do is know whether or not you have any sources applied to a particular one of your events. But here are 3 suggestions to make that whole task more "efficient".
Organize the lists on both sides by Event THEN by Source Title. So I would see a list on the left of all of the RM events I have for this person and indented under that a list of the Source Titles that have been applied to each Event. If there were no Sources yet applied to an Event I could see that. That's #1. #2 is even sweeter ... use drag and drop. Click on the Source in one pane and drag and drop it to the other pane and that would apply THAT Source Title to THAT Event. And #3 - make it possible to drag a Source to a "new" Event, one that does not yet exist in RM or (if this makes sense) in FS,
Some details - since RM can have multiple birth or death or marriage or ... events, the list on the left under RM needs to not just say Birth, but actually say Birth - date - place. And if you click on the Event line (instead of on one of the Sources under it) you get to edit the Event ... change the date or place and (if this makes sense) change the Event type. I have lots of Residence events imported from FS that really should be Census events and I have to go back after the fact and correct those one-by-one.