Psychoduck of Forge Epidemic walks you through how to set up 1 Flag CTF in Halo 4 in this video tutorial. The link to download a 1 Flag gametype, by Flying Shoe ILR, can be found below.
One Flag in Halo 4?!? Sweet! Take me to the download! >>
Ok, I got the 1 Flag gametype, now how about a 1 Flag BTB map?!? >>