Start with full ring flies with straighter arms, then go into bent-arm flies before finishing up with push-ups.
If full flies are too challenging, just start with the bent-arm flies. Warning: Your chest will hate you for at least a few days after just one set of these.
Remember, a drop set isn't a race against the clock, so don't rush the reps for the sake of finishing quickly. You want to move quickly between exercises, but don't rush the reps themselves.