Lead is a very important item that is used to tie passive mobs with fences.
To make Lead, you will need Spider Strings and Slime Ball. Please follow the following steps to make Lead.
In order to make lead, the basic item that you need to get is spider strings. You can find spiders in caves or in fields at night time, kill them using your iron sword to get at least four strings.
After that you will need to find and kill slimes. Look for the slimes in swamp biomes and kill them to get a slime ball. You need to be very active to kill them because they bounce a lot.
Now you will have to use the crafting table to make a lead. Go towards the crafting table and open 3*3 crafting grid to add four strings and a slime ball in the same way as shown in the image below. You will get lead in the right-side box of the crafting table.