There's a bit of squirming going on as to how they are going to accomplish it. Local authorities in England are, I believe, forbidden to open new schools. New schools have to be "Free Schools" which are directly funded by the DofE.
So they could give permission for organisations to start "selective free schools" and give them lots of money. A huge amount of money has already been squandered on free schools, including cases where the DofE has apparently paid way above market rents for buildings. Many of the buildings are makeshift and unsuitable.
I think most people associate grammar schools with good quality buildings, with excellent facilities for sport and all academic subjects. I remember talking to a man once who'd been to an English grammar. When I said that the Welsh girls grammar I'd been to was housed in draughty wooden sheds and certainly no playing fields he was incredulous.
Many English boys grammars had fantastic facilities - like the one DH went to, which had been going since Tudor times.
However it is unlikely that any completely new grammars will have the necessary land and money... And the DofE would have to oversee the whole process.
The only other way to achieve this is to allow some schools - presumably "academies" that are already achieving very well - to become selective. Which would be a very gradual process, a year at a time.