In terms of actually visiting the site, I should point out that it's in a private paddock, part of a large estate, I believe, at the back of the church. When I visited, the paddock was shared by a Shetland pony and a beautifully pristine white goat (ideal for sacrifice?).
To reach the stones: there is a wooden gate on the main B3095 just before the church if you are coming from Longbridge Deverill. Go through the gate into a small area of scrub land (I assume this is private, so obviously tread lightly), through which various paths meander up to the fence around the paddock containing the stones. That's where I took the picture from.
There is also a more distant view of the stones from just along the side road to Maiden Bradley, and a private lane that goes to the large house.
I've been searching for these sarsen stones for some time...
The story in the area (see Folklore) is that these marked the place where King Alfred addressed his forces before the Battle of Edington.
They are believed to be the remains of a chambered mound, with the back and cap stones assumed to have been nabbed by a farmer. And on that theme, a Wiltshire Archaeological Magazine of 1877 states "certain large stones were examined: they are called 'Egbert's Stones' or 'King's Stones' and are spoken of by the Saxon Chroniclers; they were brought by a farmer from King's Court Hill, where King Egbert is traditionally said to have held court…"
('Kingston' may mean 'King's stone', or may not.)
Terence Meaden (albeit a controversial figure) has been campaigning for the stones to be returned to Court Hill (ST835368), where he believes the original chambered mound lay. (The area is rich in long barrows and tumuli.)
The story of Egbert's Stone is an early English rather than a megalithic one, of course: in 878ce, King Alfred rallied armies from three counties (Somerset, Wiltshire and Hampshire - apparently the Dorset men were occupied at the coast) to fight the Danes.
Tradition has it that they met at Egbert's Stone - and there are at least three different sites claiming to be that of the stone (others being a three-counties boundary post at Bourton, and the site of the 18th century folly King Alfred's Tower - both are a few miles from Kingston), although it seems that the Kingston Deverill site (or, rather, Court Hill, where the stones are assumed to have come from) is regarded as the strongest claimant.
After the armies met, they marched to Ethandun (Edington) and defeated the Danes - somewhere near the White Horse of Westbury/Bratton.