Find a pumpkin
Craft shears from 2 iron ingots
Use the shears on the pumpkin to carve it
Craft a torch from 1 stick and 1 coal or charcoal
Open the crafting table
Place the carved pumpkin in the center slot
Place the torch in the slot directly below it
Take the pumpkin lantern from the result slot
Place the pumpkin lantern wherever you want to light up an area