[susan53]

No, this type of question must have a subject. How/what +to etc are incorrect - don't use them. These structures are generally used in subordinate clauses. So the following are correct :

I don't know how to do it.
She told me what to do.
Can you tell me how to do it?

but *How to do it? is incorrect. The correct question uses an auxiliary verb and subject :

How do you do it?
How should I do it?

The structure may also be used as a heading for a section which is going to give you the answer : How to book your flight /When to visit Milan. But these are not questions - they are really a short form of the idea : The next section will tell you how to book your flight - so we're back to the idea of a subordinate clause again.

The only time I could imagine the structure being used in a question would be if the question was rhetorical : Where to draw the line? That's the problem that we all face. But I've not been able to find any authentic examples. However, I can't imagine it ever being used as a genuine question - and certainly haven't been able to find examples of that.

True "questions without a subject" are those where the question word replaces the subject. So :

Who wrote this report?
David wrote it.

As you can see, Who refers to David - the subject of the answer. So in the question Who = subject. In this type of question there is no auxiliary verb - you just use the "ordinary" verb form and word order.