I am thinking of trying a FF string on my take down green tips. Does anyone have any suggestions on how to go about it?

If the bow was not made for FF then using it is one of the easiest ways to ruin it. Some people do it and get by at least for a while and then I have seen one destroyed in just a few shots.
You could always have the old tips redone with reinforced material so you could safely use FF. That is if you do it before they are damaged.

I've been using padded fast flite (D-97) on my red tips for three years.  No problems.