Hi i went to Kerala this week so I can share my experience with you...first you should visit kumarokom rather than going for Alleppey as it has backwaters too but in kumarokom you may get good resort to stay..you can land in Kochi and book a taxi for how many days u want then go wherever you can.. mostly munnar and Alleppey you do at a time.. avoid August month as its may raining there
August might be rainy. I recently took a short trip to Kerala, and I must tell you Varkala and Kovalam beach are must tries. Try and stay at these places for at least one or two nights for the maximum experience. I did not get time for visiting the backwaters, but as much I've heard it's also a must try. Regarding your itinerary, you will find plenty videos in YouTube.