The parameters of fantasy football playoffs aren't standard from league to league, so always be sure to check your league settings.
In many instances, they run from Weeks 15-17, with six teams getting berths and the top two seeds receiving byes Week 15. In this playoff format, the 3rd seed competes against the 6th seed, while the 4th seed plays the 5th seed in Week 15. The winners advance to Week 16, where one competes against the top seed and the other competes against the 2nd seed. The winners of those two semifinal matchups move on to face off head-to-head in the championship game during Week 17.
In leagues in which just four teams make the postseason, a common setup uses the combined scores from Weeks 15 and 16 to determine semifinal winners, before taking the combined Week 17-18 score to figure out the champion.