To make a Potion of slowness (-15%;1:30), you first need to make an awkward potion
After the awkward potion is finished, you have to make a swiftness potion
after the swiftness potion is finished put a fermented spidereye in the top slot and wait for the slowness potion to finish
For the slowness IV effect (-60%;0:20), put a piece of glowstone dust in the top slot and the slowness potion (1:30) in the bottom slot
For a longer lasting effect (-15%;4:00), put a piece of redstone in the top slot and the slowness potion (1:30) in the bottom slot
If you want to make a splash Potion out of it you simply put a piece of gunpowder in the top slot
And if the splash potion is not enough for you and you want to make a lingering potion from it, then you just put a bottle of dragon´s breath in the top slot and the splash potion in the bottom slot
