In Letter 20, senior hell-demon Screwtape advises his nephew Wormwood (junior demon-in-training) on how to sexually tempt the human being that Wormwood has been assigned to work on. Screwtape notes that the human being in question isn't susceptible to the more obvious sexual sins such as promiscuity or marital infidelity. Screwtape advises his nephew to instead focus on tempting the man into marrying the wrong type of woman: a flighty, insecure, materialistic type of woman. This will set the stage for an unsuccessful marriage, leading the human away from chastity and love.
In Letter 21 Screwtape talks about how Wormwood can keep the human in an ill-tempered mood. The man tends to become frustrated and short when people interrupt him, or make demands on his time. Screwtape advises Wormwood to use this to his advantage. He encourages Wormwood to keep the man deluded with this sense that all his waking hours somehow "belong" to him.
The reason this is a false notion is that, in fact, man no more earns or makes the 24 hours in his day than he earns or makes the stars, the moon, or the sun. Screwtape tells Wormwood to never let the man discover that "his time" is just as much a gift from God as the stars, moon, and sun. In doing so, Wormwood will ensure that his human becomes ever more selfish, attached, and possessive.
In Letter 22, Screwtape expresses displeasure at the fact that the human has fallen in love with a Christian woman of strong, enduring faith. Screwtape admonishes Wormwood that his hold on this human charge is weakening. Screwtape then advises Wormwood to increase his tactics in order to bring the human firmly under demonic influence. He also becomes angry toward Wormwood. He warns that if Wormwood isn't successful with this human, the junior demon himself will be brutally punished in hell.