The narrator had many characters he used in different group meetings. Marla meets him as Cornelius for example. When the narrator was in the presence of another faker, he couldn't let go, he couldn't cry, and so he couldn't sleep. Marla was a constant reminder, he was a fake as well.
He tried to explain how essential these support group meetings were to him, but she didn't care. Marla faces her problems in a very head on way. She goes to meetings to see people die, so she can feel alive. She cares very little for what everyone thinks of her, openly smoking in a cancer support meeting.
They split up the groups, and exchange phone numbers, but the narrator had long been without rest due to Marla, and he departs soon after in a business trip, where he meets Tyler for the first time who gives him his business card with his phone number. We had seen Tyler before, when the narrator was extremely tired and complaining of lost time. Tyler'd walk by the narrator, or just flash across the screen.
Looking back at one of the meetings, they are taught to go inside their cave, and find their power animal, which represent themselves. The narrator, initially has a penguin, but later his power animal becomes Marla. The narrator refuses this - or at least he thinks he does. But by obsessing with her, he gives rise to Tyler, and Tyler and Marla have plenty in common. He created Tyler in the image of Marla.
When you examine Tyler, you can see Marla in the way he thinks and preaches. He said stop caring what other people think. To do what you need to feel alive. And to stop trying to control your life. So, in short, Marla was the inspiration for Tyler. And Tyler is the bigger than life / taken to the extreme version of Marla's way of life (at least in the eyes of the narrator).