If your brother sins against you, go and tell him his fault, between you and him alone. If he listens to you, you have gained a brother.
Before anyone can admonish another, they must first reflect on themselves. Use an examination of conscience and make an act of contrition, resolving to sin no more and to grow in virtue.
More immediate than larger entities and structures, we are surrounded by brothers and sisters who, like us, often fall into sin. When someone else commits injustice and sin, have the courage to reprimand them, with love and patience. Do not say nothing when you witness evil and injustice. Call others out on blasphemy, heresy, gluttony, racism, lust, gossiping, wrath, or any other sin which contributes to the evil of our modern cultures.
The principle of subsidiarity must be remembered: admonish only those with whom you have a relationship. Your own friends, family, communities, businesses, countries.
The politics of the world are immeasurably corrupt and it is our Christian duty to speak out against evil. Participate in and organize marches and rallies. Picket corrupt institutions. Inquire at your parish how you can protest injustice. If there are no opportunities ask your fellow parishioners and search online for local protests and activist movements. If none exist already, establish connections between your local parish and these movements. Share these groups and their events in the parish bulletin. Get involved in your local Catholic Worker House. If none exists, start one. Sign up with our mailing list for updates. Most importantly, pray for justice.
Organize and participate in boycotts of companies which commit grave injustices.
If you are a writer or any other sort of public figure, use your platform to admonish evil and injustice.