Craft 1 lead using 4 string and 1 slimeball
Place the 4 string in a crafting table in a diamond shape around the center
Put the slimeball in the center slot
Collect string from spiders, cobwebs, or chests
Collect slimeballs from slimes or swamp biomes
Use the lead on animals or mobs to attach them
Right-click the mob again to detach it