Are there any medieval (from the 5th to the 15th century) directives (some kind of manual or rubrics) about what to do/pray/think during Eucharist Adoration?
Eucharistic Adoration as it is known today didn't exist between the 5th and 15th centuries. Even Quantore (or forty hours devotions) did not begin until the 16th century. Apart from religious houses, perpetual Eucharistic adoration did not begin in earnest with the laity until the 18th and 19th centuries in Italy and France. (Wikipedia)