You have two ways to do. You can buy a t-bar diffuser like this:
You will then need the appropriate collar to attach to it (corresponds to flex duct size i.e. 6", 8" etc.). This is the preferred method.
You can also just cut a boot into a ceiling tile. I usually use a bracket system that spans beyond the grid to help support the boot. Most of the time is just use boot rails. You will then buy a ceiling register to screw to it. This will hold it in place. You can buy boots, flex, straps, tape, etc. and big box stores. You will have to buy the commercial diffuser from a supply house.