I've found it the easiest to start as far back as possible, merge the oldest ancestors and then their chldren/spouses, etc. Then go to the next and do the same.
Slowly, cautiously, be aware of every step you take. Unless they are your own profiles, you are trampling on someone else's entries and you must be correct.
If they are not obviously the same profiles, do an unmerged match, this is easier to fix than a mistake. If you're not sure, don't do anything.