What prayers are said when reposing the sacrament?
Usually (but not always) reposition of the Blessed Sacrament begins with singing the hymn Tantum Ergo.
At the end of the hymn, the celebrant stands and says/chants while the people respond:
Celebrant: You have given them bread from heaven.
Congregation: Having all sweetness within it.
Celebrant then prays: Lord Jesus Christ, you gave us the Eucharist as the memorial of our suffering and death. May our worship of this Sacrament of your body and blood help us to experience the salvation you won for us and the peace of the kingdom where you live with the Father and the Holy Spirit, one God, for ever and ever. AMEN.
The celebrant takes the monstrance and gives the blessing. Afterwards, he places the monstrance on the altar.
The celebrant then leads the congregation in the Divine Praises:
Blessed be God.
Blessed be His Holy Name.
Blessed be Jesus Christ, true God and true man.
Blessed be the name of Jesus.
Blessed be His Most Sacred Heart.
Blessed be Jesus in the Most Holy Sacrament of the Altar.
Blessed be the Holy Spirit, the Paraclete.
Blessed be the great Mother of God, Mary most holy.
Blessed be her holy and Immaculate Conception.
Blessed be her glorious Assumption.
Blessed be the name of Mary, Virgin and Mother.
Blessed be Saint Joseph, her most chaste spouse.
Blessed be God in His angels and in His Saints.
Immediately following the Divine Praises the celebrant reposes the Blessed Sacrament.