Each world was given a symbol. These symbols were woven into the magic used to create the doors and only people with a tattoo of the symbol could use them. At first they were only given to the walkers, who had gone through years of study in order to get prove themselves worthy. Some only had one tattoo, focusing in depth on one specific world; others had multiple tattoos, normally a maximum of ten, but there were walkers who had more; and those with more than twenty tattoos were automatically given a place on the World Walkers council.
Later the magic was adapted, so that others could travel around the web. It wasn't something that many people did, because getting a temporary tattoo was expensive, but this did begin the merchant trade around the web.