The Legendary maps are Ghost Town, Blackout, and Avalanche.
I haven't play a CTF game on ranked or social setlists that weren't on Avalanche. To help, I'd suggest playing on the DLC BTB playlist. However, CTF seems to come up very rarely.
Anyway....if you happen to be playing on Avalanche and it's a CTF game type, you're in luck! You're goal is to acquire the flag and kill someone with it. I would suggest taking the teleporter from your base to the mancannon by the outside edge of the map. Use it to get to the middle and move towards the other mancannon straight ahead (you're going to be going to the other teams teleporter room to get inside of their base).
Quickly grab the flag (crouch walk if you got to) and make a run for the teleporter. Once you go through the teleporter, move to the side and wait. Chances are, at least one guy will follow you. When they come out and walk past you, hit them with the flag. It should be an instant kill and you should get your achievement a few seconds after the kill has been made.
So, to summarize (CTF on avalanche)
1) Teleport from your base to mancannon
2) Mancannon to middle
3) Walk/use ghost to other team's side mancannon
4) Use the teleporter
4) Grab flag and go through teleporter
5) Move to the side and camp
6) Assassinate anyone who follows you.
Obviously you can get this kill any other way, since the flag is an instant kill weapon, but you also move extremely slow when carrying the flag, so it's best not to charge at someone with it.