How do I make an act of contrition?

Perfect and imperfect

Contrition arising from love of God above all is "perfect." It forgives venial sins, and mortal sins too if it includes the firm resolution to go to confession as soon as possible; it does not remove the obligation to confess. "Imperfect" contrition, born of sin's ugliness or fear of punishment, is also God's gift; it cannot by itself forgive grave sins but disposes one to receive forgiveness in confession (CCC 1452-1453).

The prayer

An act of contrition expresses the penitent's sorrow to God. No formula is required; it can be as simple as "Lord Jesus, Son of God, have mercy on me, a sinner." The Handbook of Prayers offers a traditional form, expressing heartfelt sorrow for offending God — out of fear of losing heaven and of hell, but most of all because sin offends God, who is all good — and resolving, with his grace, to confess, do penance and amend one's life.