I think its clear from scripture we are to teach the word in season and out.
2 Timothy 4:2
2 preach the word; be ready in season and out of season; reprove, rebuke, exhort, with great patience and instruction.
We are commanded to tell people to not teach falsely.
1 Timothy 1:3-5
3 As I urged you upon my departure for Macedonia, remain on at Ephesus so that you may instruct certain men not to teach strange doctrines, 4 nor to pay attention to myths and endless genealogies, which give rise to mere speculation rather than furthering the administration of God which is by faith. 5 But the goal of our instruction is love from a pure heart and a good conscience and a sincere faith.
But more than that, we are to expose deeds of darkness.
11 Do not participate in the unfruitful deeds of darkness, but instead even expose them;
I think 1 Timothy 5:19-20
19 Do not receive an accusation against an elder except on the basis of two or three witnesses. 20 Those who continue in sin, rebuke in the presence of all, so that the rest also will be fearful of sinning.
ties in with Christ’s teaching in Matthew 18:15-17
15 If your brother sins, go and show him his fault in private; if he listens to you, you have won your brother. 16 But if he does not listen to you, take one or two more with you, so that BY THE MOUTH OF TWO OR THREE WITNESSES EVERY FACT MAY BE CONFIRMED. 17 If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector.
Both of these tie back to
6 “On the evidence of two witnesses or three witnesses, he who is to die shall be put to death; he shall not be put to death on the evidence of one witness.
and Deuteronomy 19:15
15 “A single witness shall not rise up against a man on account of any iniquity or any sin which he has committed; on the evidence of two or three witnesses a matter shall be confirmed.
I think the first thing to do is pray. Then to go to the person in private, teaching from scripture what the right thing to do is. If they change, then our work is done. However, if that fails, we are pray and to take two or three others (again showing discretion) and go to the person and show them again from scripture, perhaps using different arguments from different places in scripture. If they change, then we’re done. If not, then we need to pray and to go to the leaders. If the leaders are unable to convince the people to change, we are to pray and go to the church publically and they should be given yet another chance to change. If they do not, they should be asked to leave the church until such a time as they change.
If the problem is with the leadership of a church who are part of a hierarchy of leadership outside the local assembly, then I think the thing to do is go to them before going to the church at large. If the problem is with the leadership of a church who are not part of a hierarchy, I think the thing to do is go to them, then to the church assembly. It may be this then needs to be taken to the church at large – I think that would be a call each situation would require a separate decision to be made, and I see no scripture guiding us here. IT may be we should always go directly to the church assembly once we have gone to the individual as part of a group, regardless if they are leaders or not.
So in summary, each step in the process should be bathed in prayer. We need to go to the person, one-on-one, teaching truth from Scripture. Then we need to go to them with two or three others. Then we go to the church leadership. Then to the church. If the problem is with someone in the leadership, then when we go to him, we will still need to go to the local church leadership (deacons or other elders) before we go to the church at large. If the church is part of a hierarchy, then we might want to take it to them before taking it to the local church.