This is how they marketed Doom:
Word-of-mouth. First of all id Software realized that calling big papers and magazines wouldn't work. These magazines were not interested, in part because the game included violence not seen before. Instead they established a toll-free number to held orders and set up a deal with a fulfillment house that would package and send Doom to whoever ordered it. Id Software already had an audience who wanted to pre-order the game. Having a small initial audience is always good because they will talk about the game with their friends and so the game will market itself with the help of word-of-mouth.
Shareware. Id Software decided to self-publish Doom to make more money: they would make 85 cent for every dollar sold. To self-publish they decided to go shareware, so they would give away the first part of Doom for free and if a customer wanted more of the game the customer would have to pay for the rest of the game. This will increase the probability that a customer will test the game because the customer is not forced to pay for what's maybe a bad game.
Let someone else pay for the paid marketing. To distribute the shareware id Software contacted retail stores. This was before fast Internet speeds, so even though some customers downloaded the Doom for free from the Internet, all customers couldn't just download the game. Instead they had to buy it. At the time, retail stores were selling shareware and were forced to pay a royalty to the creator of the shareware because the store made money from the otherwise free shareware. But id software decided to not demand a royalty from the retail stores, telling them:
"Take Doom for nothing, keep the profit! My goal is distribution. Doom is going to be Wolfenstein on steroids, and I want it far and wide! I want you to stack Doom deep! In fact, I want you to do advertising for it too, because you're going to make money off it. So take this money that you might have given me in royalties and use it to advertise the fact that you're selling Doom."So the retail stores would pay for all the paid marketing of the game and id Software wouldn't have to pay a single dollar for it. But id software decided to pay for one small ad in a gaming magazine - you can't get everything for free.