I have never got $100 a day from affiliates marketing, but I have earned $89 from that. I think the basic method is that you choose the right product which have high commissions (about $40).
Then, you get traffic to your site by any ways such as Google Search Engine, Online advertising and so on.
You will earn $100 if your site have 3 sale in that day. You also can create more sites for more products. I think $100 a day is ability.