Slack key is a style, not a tuning. You do not need to tune your ‘ukulele differently, but some artists do. Uncle Ledward Kaapana sums it up nicely: “When I was young my uncle Fred told me you can play slack key in Standard tuning. He said, ‘It’s easy, jus’ press the right strings’. Jus’ press’ was something he would always tell us when we’d ask him a question. One time when we were playing I asked him, ‘Uncle Fred, what key is this?’ He told me, ‘Boy, no worry what key, jus’ press.’”
I am trying to find some slack key in the Key of A to use in some Blues songs. Actually, I think all I need to do is write down the A scales and copy the methods to do slack key in C.