Once the top tray that you have been adding scraps to becomes ¾ full, you can begin to rotate. If you have just started, you will have an empty tray (with maybe a few castings that have fallen through) sitting below your top tray. 

Take the empty tray and move it to the top. You can also bring across any food scraps to the new top tray that are taking longer to break down i.e. avocado seeds or corn cobs. 

The full tray will begin to break down while the top tray begins to fill up. If you find that your top tray becomes full before the castings in the bottom tray have fully developed, you can always add an extra tray into the rotating cycle to give it extra time to break down.