It is easy to recognize in our ordinary Benediction service, the traces of two distinct elements. There is of course in the first place the direct veneration of the Blessed Sacrament, which appears in the exposition, blessing, "Tantum ergo", etc. But besides this we note the almost invariable presence of what at first sight seems an incongruous element, that of the litany of Loreto, or of popular hymns in honor of Our Lady. Tracing our present service back to its origin we find that these two features are derived from different sources.
The idea of exposing the Blessed Sacrament for veneration in a monstrance appears to have been first evolved at the end of the thirteenth or the beginning of the fourteenth century. When the elevation of the Host at Mass was introduced in the early years of the thirteenth century, probably as a form of protest against the theological views of Peter the Chanter, the idea by degrees took firm hold of the popular mind that special virtue and merit were attached to the act of looking at the Blessed Sacrament. To such extremes did this prepossession go, that the seeing of the Host at the moment of the elevation was judged to be the most vital part of attendance at Mass. On certain churches in Spain a screen of black velvet was held up behind the altar in order that the priest's hands and the Host might be more easily seen from afar; in others strict injunctions were given to the thurifer that he should on no account allow the smoke of the thurible to obstruct the view of the Host. Furthermore, we read that when men were dying and were unable on account of vomiting or any other cause to receive Holy Viaticum, the Blessed Sacrament was brought to them and held up before them to look at. Indeed, a virtual prohibition of this practice stands to this day amongst the rubrics of the "Rituale Romanum."
Under the influence of this idea, the Blessed Sacrament in the processions which became common after the institution of the feast of Corpus Christi in 1246, came by degrees to be carried in transparent vessels, resembling our present monstrances. Moreover, a custom grew up, especially in Germany, of keeping the Blessed Sacrament continually exposed to view in churches. It was forbidden by many synods, but a sort of compromise was arrived at through the construction of the
Sakramentshäuschen of which so many examples still exist in central Europe. These tabernacles, of great height and imposing appearance, were erected in the most conspicuous part of the church, and there the Blessed Sacrament was reserved in a monstrance behind a metal door of lattice-work which allowed a more or less free view of the interior. It was thus that the practice developed, though partly kept in check by synodal decrees, of adding solemnity to any function, even the Mass itself, by exposing the Blessed Sacrament during its continuance.
Turning now to our second element, we find that from the beginning of the thirteenth century, a custom prevailed among the confraternities and guilds which were established at that period in great numbers, of singing canticles in the evening of the day before a statue of Our Lady. These canticles were called
Laude and were often composed in the vulgar tongue, becoming in the hands of such poets as the Franciscan Giacopone da Todi, one of the great popular influences which helped to develop a native Italian literature. Confraternities were formed for the express purpose of singing these canticles and their members were called
Laudesi. It was such a company of
Laudesi that brought together the seven holy founders who, in the first half of the thirteenth century, established the Order of Servites, or Servants of Mary. Although the
laude hardly flourished outside Italy, where both the language and the character of the people lent themselves readily to the composition of innumerable canticles, the idea of an evening service of a popular character sung before the statue of Our Lady, spread throughout Europe. In particular, the "Salve Regina", a special devotion of the Servites, Dominicans, Carmelites, and other orders, was consecrated by usage to this rite, and we find traces everywhere of its being sung, often by choirs of boys, for whom a special endowment was provided, as a separate evening service. In France, this service was commonly known as a
Salut, in the Low Countries as the
Lof, in England and Germany, simply as the
Salve. 
Now it seems certain that our present Benediction service has resulted from the general adoption of this evening singing of canticles before the statue of Our Lady, enhanced as it often came to be in the course of the sixteenth and seventeenth centuries by the exposition of the Blessed Sacrament, which as employed at first only as an adjunct to lend it additional solemnity. The blessing at the close seems to have been added simply because the custom gained ground of making the sign of the Cross over the people whenever the Blessed Sacrament was replaced in the tabernacle after a procession or after being carried to the sick or any kind of an exposition. But in the course of the seventeenth century, we find numberless bequests for
Saluts in French wills, the items to be sung, often of a most miscellaneous character, being minutely specified, and among these the condition is frequently appended that the Blessed Sacrament should be exposed during the whole time of the
Salut.
Reference: http://www.newadvent.org/cathen/02465b.htm
After the incensing the priest prays the Collect of Corpus Christi, then stands and dons the humeral veil, ascends to the altar and lifting the monstrance above his head traces a large cross.
