How to do is asking for instructions. For example, if somebody asks, "Can you tell me how to make myself a bowl of cereal?" you would reply with something detailed, like, "To make cereal, get the bowl and pour the cereal, then pour the milk." However, with what to do, is simply to do it. Like asking, "Can you tell me what to do?" and you would answer saying something similar to, "Make cereal." Unlike how to do, it is very vague and does not include as much detail or the same exact instructions. Hope this helped!

Gotcha. Tom. I really appreciate your help. It's useful for me that easy to understand.

"Tell me what to do." You should make a peanut butter and jelly sandwich. "Tell me how to do that." Get two pieces of bread. On one, spread peanut butter. On the other, spread jelly. Put the pieces of bread together so the peanut butter and jelly meet in the middle! Native English speakers will never say, "How to do" by itself - 'How to..." needs an object. How to do SOMETHING. How to make a sandwich. How to run. How to speak English.