The diabolical Screwtape really doesn’t want you to pray, and he’s got some great ideas to keep that from happening.
The best thing, where it is possible, is to keep the patient from the serious intention of praying altogether. When the patient is an adult recently reconverted to the Enemy’s party, like your man, this is best done by encouraging him to remember, or to think he remembers, the parrot-like nature of his prayers in childhood. In reaction against that, he may be persuaded to aim at something entirely spontaneous, inward, informal, and unregularised; and what this will actually mean to a beginner will be an effort to produce in himself a vaguely devotional mood in which real concentration of will and intelligence have no part. One of their poets, Coleridge, has recorded that he did not pray ‘with moving lips and bended knees’ but merely ‘composed his spirit to love’ and indulged ‘a sense of supplication’. That is exactly the sort of prayer we want; and since it bears a superficial resemblance to the prayer of silence as practised by those who are very far advanced in the Enemy’s service, clever and lazy patients can be taken in by it for quite a long time. At the very least, they can be persuaded that the bodily position makes no difference to their prayers; for they constantly forget, what you must always remember, that they are animals and that whatever their bodies do affects their souls. It is funny how mortals always picture us as putting things into their minds: in reality our best work is done by keeping things out. …
But of course the Enemy will not meantime be idle. Whenever there is prayer, there is danger of His own immediate action. He is cynically indifferent to the dignity of His position, and ours, as pure spirits, and to human animals on their knees He pours out self-knowledge in a quite shameless fashion.
The issue for Lewis here is the idea that prayer is somehow a purely internal feeling instead of a thoughtful process involving our whole being. For him, being scheduled and disciplined in prayer aids in having deep and meaningful devotion. Spontaneity may feel good, but it often results in shallowness.
Lewis was an Anglican, and as such, he would’ve been familiar with the Book of Common Prayer, and likely used it in his personal devotional life on a regular basis. Several years ago, I began using prayer books in my own devotional life, and have found them to be very helpful and focusing. I would encourage you to investigate these and see if you find them helpful in your prayer life.