The process to create a Skaill knife is very simple.
Throw a suitable beach cobble against another secure stone (but stand back – they can bounce!). The resulting flake has a sharp edge, providing a cutting tool that experimental work has shown to be highly efficient at butchery .
It was the Orcadian antiquarian George Petrie who recognised Skaill knives for what they were. In 1867, almost 20 years after Skara Brae was exposed by the elements, Petrie wrote: “There is one type of stone implement very abundant at Skaill.” 
These flakes had been:
“…broken off from water-worn stones, and have invariably one side smooth, while the other shows the fracture. One edge is always thicker than the other, which is almost invariably very sharp.” 
“There were hundreds of those flakes found at Skara, but not one has received the slightest polish, and very few of those found seem to have been used, as most of their edges are as sharp as if the flakes had been newly detached.” 
Although certain the flakes were not the result of natural attrition, Petrie was perplexed. “The process by which these rude stone implements had been obtained seemed to be still a matter of doubt,” he declared.
The answer came on a visit to Westray, when Petrie witnessed his son hurling beach stones on the rocks. The boy, he realised, had “unwittingly rediscovered the ancient mode of producing the rude stone implements”.
Petrie had been investigating a “kitchen midden” within some “artificial mounds”. Excavating with a pocket-knife, he and a colleague found “great numbers of stone flakes similar to those from Skara”. 
The artefacts were clearly on his mind when he later encountered a stone on the beach. It had clearly been broken a few hours earlier, but by natural causes.
“On lifting it, I saw, with some surprise, that a flake of a circular shape had been recently struck from it, and at the upper edge, where the stroke had taken effect, was a notch, the counterpart of those by which the flakes found at Skara are characterised.”
Watching the results of his son’s stone-breaking exploits, Petrie had a go himself:
“I dashed the broken stone, which I still held in my hand, on the rocks and as I confidently expected, and to my great delight, a flake was detached as good as any found at Skara.
“I frequently repeated the experiment, and with invariable success; and I have little doubt that such was the simple mode by which the flakes of the ancient kitchen middens in Orkney were generally obtained.
“To break them off with another stone wielded in the hand, would be a process both tedious and uncertain in its results. I believe a much more powerful stroke than can be given in that way is necessary to produce a notch like the one which is always made by dashing the stone on the rocks, or on another stone of sufficient size and hardness to resist the blow.” 
The nature of their fabrication means that Skaill knives can vary in size, with their shape dictated by that of their “parent” cobble.
“The typical Skaill knife is … often squat to round in shape with a thick proximal end that enables it to be gripped comfortably in the palm of the hand, leaving free a long, unmodified working edge. No physical evidence remains to indicate that these flakes were ever hafted prior to their use.”
Few excavated examples show evidence of reworking – it seems they were used until the sharp edge dulled, or was damaged, and then thrown away.
“The Skaill knife can therefore be regarded as a tool that is quickly and easily produced from an almost infinite stone resource. There is very little evidence for the curation of these flakes, and they can be viewed as highly disposable items, sometimes found in great numbers, which may well have been used only once before being discarded.”
As mentioned previously, it seems likely that Skaill knives primary role related to butchery. Evidence gathered by lithics specialist Ann Clarke has shown that, for this purpose, they were extremely efficient:
“The modern-day butcher uses a selection of knives and choppers of varying shapes and sizes; this was reflected in the choice of flakes in [an] experimental butchering session where the butcher selected the smallest flakes for skinning and the largest for chopping bone, with intermediate sizes used for a wide range of tasks.,
“The shape and size ranges of the [Skaill knives] found in the prehistoric assemblages would easily accommodate this need for selectivity in tool shapes and sizes.”
 Clarke, A. (2006) Stone tools and the Prehistory of the Northern Isles (Vol. 406). British Archaeological Reports Limited.
 Petrie, G. (1867) Notice of ruins of ancient dwellings at Skara, Bay of Skaill, in the Parish of Sandwick, Orkney, recently excavated. In Proceedings of the Society of Antiquaries of Scotland (Vol. 7, pp. 201-219).