Each Sign of the Cross (in the form of a blessing) is dictated by the rubrics of the Mass contained in the Missale (Roman Missal). Most are not blessings as you can only bless something once (humans excepted). These are symbolic gestures that have been canonized in the Mass representing the Sacred Trinity, for the most part. There are many excellent books that explain all of the symbology of the Traditional Mass. I would encourage reading these books to appreciate the beauty and significance of each action. None of them are ad-libbed and each has a significance. (The Mass of All Time, +Marcel Lefebvre, THE Mass, A Study of the Roman Liturgy, Fr. A. Fortesque.)