i want to know how to do和i want to know how to do it.

IT 是不能缺少的,虽然你用中文直译可以我想知道怎样去做,但是在英文里面是说不通的,首先如果没有IT就没有了宾语,这不是个完整的句子,再说如果没有IT 也不知道你指的是什么,不可能在 没有遇到什么事情是不可能会说这句话的.这是个名词性定语从句,HOW 修饰的就是IT 来解释i want to know.

I want to know what to do
I want to know how to do it.

I want to know 是一句
而HOW TO DO IT 是另一句
没有IT 就是缺少了从句里的成分 了

I want to know what to do .(to do what,what作宾语)

I want to know how to do it.(it作宾语)

一定要有it.
否则应该用
i want to know what to do

一定要有it.
否则应该用
i want to know what to do