He didn't create it, it's just his unique MS justu, the same way that Itachi has Tsukiyomi, Obito has kamui, and Sasuke has enton release.
Shisui was a young adult jonin with his own team and his father was the same age as Hiruzen. He was also stated to be older than Itachi. He was easily at least 5-9 years older. Considering that both Itachi and Obito can use their MS techs with either eye (well, Obito has one eye but you get the point), Shisui obviously used it once or more at one point. The eye he gave Itachi wasn't charged yet at the time of the massacre, so it's pretty obvious that he used KA in that eye sometime before. (He wanted to use one eye to do the KA, Danzo stole it and he was left with the other...but he didn't bother to attempt it again, nor was Itachi able to use it during the massacre or his fight with Sasuke, meaning Shisui really did use KA in that eye beforehand for some other purpose and it was on cool-down.)
Also, Uchihas do use their MS jutsus like pros the first time around somehow. Obito managed to slip through with kamui against those Mist ninja and he had just awakened it. Itachi used Tsukiyomi like a pro shortly after Shisui's death. Probably the same for Shisui.
