Please Say/Whisper The Words Out Loud
Lesson 1
One of the most important rules to remember when you are tempted to criticize someone or speak negatively about them is: be clear in your mind exactly what you want this person to do or to stop doing. Then figure out the best strategy to move them in this direction. This point is crucial to master and internalize.

Lesson 2 - In answer to your questions...

Q. what do you do if you have already transmitted the lashon hora to other people?

A. Try to convince the people you spoke to that what you said isn’t true. when that isn’t possible, ask the subject of the lashon hora to forgive you.
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Lashon Hora = Saying a derogatory/negative statement about someone else that is TRUE
Motzi Shaim Ra (Defamation of Character) = Saying a derogatory statement that is FALSE
About Whom is it Forbidden to Speak Lashon Hora - Any Individual Jew - Alive or not, - adult or child, family member (spouse, in-laws, parent, child, siblings), groups/sects of Jews, Entire Jewish Nation
Rechilus - A word used to depict someone who goes to a person and tells them what someone else has said about them.

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These lessons are derived from a few books:
Guard your Tongue: Adapted from Choeftz Chaim by Zelig Pliskin
The Power of Words by Zelig Pliskin
Chofetz Chaim: A Lesson a Day - The Concepts and Laws of Proper Speech Arranged for Daily Study By Shimon Finkleman and Yitzhok Berkowitz
Purity of Speech: A short lesson on the halachot of shmirat Halashon
Chofetz Chaim - A Daily Companion: Arranged by Reb Yehuda Zev Segal