Materials:
4x String
1x Slimeball
Steps:
Open your crafting table: You'll need a crafting table to assemble the lead. If you don't have one handy, use wood planks (any type) to craft one.
Arrange the ingredients: In the 3x3 crafting grid, follow this pattern:
Place two strings in the first two boxes of the top row.
Place one string directly below them in the first box of the second row.
In the second box of the second row, put your slimeball.
Finally, add the last string in the bottom right corner of the grid (the third box of the third row).
Craft the lead: Once you've positioned everything correctly, you'll see the lead appear in the crafting output box on the right. This recipe actually makes two leads, so you'll be well-prepared in case you lose one.
Drag the lead to your inventory: Move the crafted lead from the output box to your inventory for safekeeping.
Now you're all set! You can use the lead to leash passive mobs like cows, sheep, wolves, and even foxes.