The Demon has to approach the mortal in question and offer him one or more supernatural gifts. These can range from curing a disease, to special insight into the mysteries of the world to wealth and charisma, but it has to be in the range of power of the respective Fallen (no demon is, for example, able to resurrect a loved one). Most Fallen take the time and observe their potential targets for a long period to find what would be of value to them.
In exchange for these gifts, the mortal has to worship the Demon on a regular basis, offering his faith each morning. This Faith can be directly claimed by the Fallen, but some use it to enhance the Thrall's abilities, making him a more useful agent. Often, these Thralls find themselves drawn into more pacts once they had a taste of the infernal power a Demon is able to grant. Most demons use their innate abilities to fool mortal, appearing as angels to devout christians to persuade them to sign in pacts. Exact words can also be deceiving to the Thrall, as most Demons are masters in finding loopholes in these agreements to garant themselves a lion's share of the Faith.
A Pact can only be sealed after both parties agree to its tenets. Most Demons seal the Pact with a short ritual correspondending to their House (Sex for the Defilers, ritual cuts for the Slayers etc.).