M E S  

M' E' S'

Good edges can be solved by "natural" moves (R L U D). Front and Back slice turns are not natural.

Bad edges always exist in (multitudes) of pairs.

You can change 4 bad edges into good edges in the Front slice by an F or F' quarter turn. Same for Back slice with B or B'.

If you have 2 bad edges left, bring one bad edge to the front slice with natural moves. Quarter turn gives you 3 bad edges, plus the remaining one makes 4. Bring all to the front slice, etc...