There are no easy solutions here.
The starting place is to see that you do not need to understand the whole text, rather you just need to answer the questions. In the class you always skim, but this is an exam, not a class. So it is possible not to skim. It is not a matter of correct technique, only what works and what doesn't.
I'd suggest if you do skim the whole text, you should do it for a reason. The reason here is to know very generally what the text is about (the heading may be enough), but more probably you skim to understand how the text is structured: what is the beginning, middle and end. You do this only so you can find the answers more quickly by knowing which part of the text to focus on.
One more thing. If you do skim, consider making notes (underlining, highlighting) about what each part of the text is about, so you don't waste time later.

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