This is it, in a “nutshell”.
Rubrics for Low Mass
STAND: When the priest enters sanctuary.
KNEEL: Prayers at the foot of the altar.
SIT: Epistle, Gradual, Alleluia / Tract.
STAND: Gospel.
SIT: Announcements and sermon.
STAND: Credo, genuflect at “Et incarnatus est …”
SIT: After the Oremus before the Offertory antiphon (bells are rung once).
KNEEL: Sanctus (bells are rung three times).
SIT: After Communion as priest begins ablutions (purification of chalice).
KNEEL: Last Blessing, make the Sign of the Cross as the priest blesses the congregation.
STAND: Last Gospel, genuflect at “Et verbum caro factum est …”
KNEEL: For prayers after Mass.
STAND: Until priest exits the sanctuary.
During a High Mass, the priest and choir sing the Ordinary and the Propers in Latin.
During a Low Mass, the priest recites the Ordinary and Propers. The choir and congregation may sing Latin and English hymns to accompany the liturgical action at the Entrance, Offertory, Communion and Recessional.
Holy Communion:
Please note, it is customary at a Tridentine Mass for Holy Communion to be received kneeling and on the tongue.
The communicant does not say, “Amen,” because it is included when the priest says, “Corpus Domini nostri Jesu Christi custodiat animam tuam in vitam aeternam. Amen.” (“May the body of our Lord Jesus Christ preserve your soul unto life everlasting. Amen.”)