You cannot craft a cobweb in vanilla Minecraft
Find cobwebs in abandoned mineshafts
Find cobwebs in stronghold libraries
Find cobwebs in woodland mansions
Find cobwebs in some ancient city structures
Use shears to collect a cobweb
Use a sword to break a cobweb and get string
Use commands in Creative mode to give yourself a cobweb
Use the /give command: `/give @p minecraft:cobweb`