St. Paul in his letter to the Thessalonians challenges Christians to "Rejoice always. Pray without ceasing. In all circumstances give thanks, for this is the will of God for you in Christ Jesus" (1 Thessalonians 5:16-18).
Over the centuries there have been a variety of reactions to this challenge, such as the Liturgy of the Hours and the Jesus Prayer.
However, there is an even easier way to pray without ceasing that barely requires any memorization and does not need any prayer book.
Jesus
The Catechism of the Catholic Church explains the simplest way to pray without ceasing in it section on prayer:
The invocation of the holy name of Jesus is the simplest way of praying always. When the holy name is repeated often by a humbly attentive heart, the prayer is not lost by heaping up empty phrases, but holds fast to the word and "brings forth fruit with patience." This prayer is possible "at all times" because it is not one occupation among others but the only occupation: that of loving God, which animates and transfigures every action in Christ Jesus.
All you need to do is say the name "Jesus" with prayer and love.
No complicated formula prayers and no prayer books to worry about.
Try praying Jesus' name throughout the rest of today and see what happens!