You cannot make a harness in vanilla Minecraft because it is not a craftable item
In Minecraft Bedrock Edition, use a saddle to ride horses, pigs, striders, and camels
In Minecraft Java Edition, use a saddle to ride horses, pigs, striders, and camels
Find saddles in chests, fishing loot, dungeon loot, bastion loot, or by trading with villagers
To equip a saddle, right-click or use the interact button on the animal
If you meant a modded harness, install the specific mod and follow that mod’s crafting recipe