You have to take a test for one of the ratings on your CFI ticket. I'd recommend the CFII, because it's all instrument stuff that you should be proficient in if you've been flying for the airlines (I'd still review circle-to-land and non-airline procedures like that). This way, you won't have to practice and become proficient in chandelles, lazy eights, eights on pylons and all the CFI maneuvers.