First - Make sure there is nothing under, or in, the slab like pipes or
what not.

Second - Make a cut with a circular saw and a masonary blade outlining
where you want your openings.

Third - Get out your sledge hammer and pound away.

Options - Rent/borrow a demolition hammer and/or a concrete saw. These
tools will add to the cost but may make it easier in the long run. It
may not actually be faster, given trips to rental yard, etc, if your
job is small.