How:

Simply lower your chin toward your throat and hold a moment

When:

• Most especially, any time you hear the Name "Jesus" (note that "Christ" is His title, meaning "Annointed One"; there is no need to bow the head at just the mention of the word "Christ"). Men should remove their hats when His Name is spoken; this practice is for both inside and outside of Mass (yes, if you're having a casual conversation with someone on the subway and mention His Name, you actually are supposed to bow your head, removing your hat if you are a man). ¹
• Cross yourself and bow the head when the priest and the Crucifer walk down the aisle before and after Mass. After Mass, as the priest leaves the Altar, it is also customary to pray for him. (Some make a profound bow instead at these times)
• Not commonly known and practiced: any time you hear "Father, Son, and Holy Ghost (or "Holy Spirit")" mentioned together; any time you hear the name of Mary; and, during Mass, when the name of the Saint in whose honor the Mass is being celebrated

How:

With either a fist or with the tips of the fingers, held close together, strike your chest over the heart to express regret and sorrow ²

When:

• at the Mass, formally: at each "mea culpa" during the Confiteor; at the Nobis Quoque Peccatoribus (priest); three times during the Agnus Dei; and three times during the Domine, Non Sum Dignus
• informally: at the "forgive us our trespasses" ("dimitte nobis debita nostra") in the "Our Father"; any time to express penitence inside or outside of the Liturgy

How:

Bow at the waist  (about 30^o forward)

When:

• at the Aspérges at Mass when the priest sprinkles the congregation with holy water
• when the Altar boy incenses the congregation during the Mass
• Cross yourself and make a profound bow when the priest and Crucifer walk down the aisle before and after Mass. After Mass, as the priest leaves the Altar, it is also customary to pray for him. (Some simply bow the head instead of making a profound bow at these times)
• when greeting a hierarch who doesn't have jurisdiction over you (e.g., the Bishop of a diocese other than one in which you live). As you bow, kiss the hierarch's ring. This bow and ring-kissing are only done if the Pope is not present.

How:

When:

How:

Looking at what you are genuflecting toward, kneel on your right knee for a moment in the manner of a man proposing to a woman, bringing the right knee all the way to the floor and keeping the back straight. Hold for a moment, then stand.

When:

• Genuflect toward the Tabernacle where the Blessed Sacrament is reserved, and each time you pass in front of it (except when you're in procession, such as standing in line for Communion, or returning to your seat afterward). While this should, on one level, be a matter of habit, it shouldn't be done thoughtlessly. Remind yourself when genuflecting toward the Tabernacle that you are kneeling before God. Praying mentally, "My Lord and My God" is a good habit to get into while genuflecting on the right knee. If the Tabernacle is not on the Altar, genuflect toward the Altar and the Altar Crucifix.
• Before a relic of the True Cross when it is exposed for public adoration.
• On Good Friday to Holy Saturday, after the ceremony of the Adoration of the Cross, genuflect when passing in front of the exposed Crucifix on the Altar.

How:

Kneel on both knees

When:

• any time the Blessed Sacrament is exposed, to show adoration and humility
• many times during the Mass: during the Prayers at the Foot of the Altar, after the Sanctus, after the Agnus Dei, at the altar rail, and at the Last Blessing
• during Confession, inside or, in emergencies, outside of the Confessional
• when receiving a priestly blessing, inside or outside of the Liturgy

How:

Keeping your legs together, lie down flat on the floor on your face, crossing your hands underneath your forehead forming a "pillow" on which to rest your head 

When:

How:

To paraphrase Lauren Bacall in "To Have and Have Not," you know how to kiss, don't you? You just put your lips together... but don't blow.

When:

• Kissing Crucifixes and Icons (2-D or 3-D): In icons that depict more than one person, kiss first Our Lord (His Feet, Hem of His garment, or hands), then Our Lady (her hands or veil), then the the angels and Saints. To reverence a Crucifix or icon that you can't reach too well with your lips, kiss your fingers and then touch where you would kiss.
• Kissing rings of hierarchs: see above under "Genuflection on Left Knee"
• Kissing a priest's hands: the priest's hands may be kissed when greeting or leaving him because they alone are able to confect the Holy Eucharist. They are also kissed on Palm Sunday when receiving a palm (which is also kissed). During the Mass, the priest's hands are kissed by the acolytes/altar boys.

Footnotes:
¹ The custom of bowing the head at the mention of His Name was formally written into law at the Second Council of Lyons, A.D. 1274, convened by Pope Gregory X: "Those who assemble in church should extol with an act of special reverence that Name which is above every Name, than which no other under Heaven has been given to people, in which believers must be saved, the Name, that is, of Jesus Christ, Who will save His people from their sins. Each should fulfill in himself that which is written for all, that at the Name of Jesus every knee should bow; whenever that glorious Name is recalled, especially during the sacred Mysteries of the Mass, everyone should bow the knees of his heart, which he can do even by a bow of his head."

² The Catholic Encyclopedia cites St. Augustine (A.D. 354-430) as saying in his Sermo de verbis Domini, "No sooner have you heard the word 'Confiteor' than you strike your breast. What does this mean except that you wish to bring to light what is concealed in the breast, and by this act to cleanse your hidden sins?"

It also cites St. Jerome (ca. A.D. 340-420) as saying in Ezechiel, c. xviii, "We strike our breast because the breast is the seat of evil thoughts: we wish to dispel these thoughts, we wish to purify our hearts."