The Warning illuminates the conscience of each person, showing them the sins that they have committed, as understood by their own conscience. But the Warning only shows people the sins still remaining on their consciences. So if you have repented from a past sin, it might not be shown to you.
Therefore, the best preparation for the Warning is to examine your conscience and repent and go to Confession. The Warning does not give anyone objective knowledge of right and wrong. So when the Warning shows you a sin, it is showing you an act which you yourself knew was morally wrong, at the time the act was committed.
When the Warning happens, accept the correction offered by this difficult blessing. Admit that you are a fallen sinner, who has sinned many times in life. Regret the things that you have done wrong, offenses against your own conscience. And resolve to love God and neighbor, to avoid all grave sins in the future, and to follow your conscience.
If the Warning throws you into a deep sadness, do not despair. The next event is the Consolation, on Easter Sunday. It is a blessing given to all who accept the correction of the Warning, by repenting of their sins. The Consolation relieves the suffering caused by the Warning: the sorrow and difficulty of admitting your sins. The Consolation consoles and strengthens the human spirit, so that we are not overburdened by the realization of our sins.
If you find the Warning on Good Friday to be overwhelming, consider your sins, repent, and resolve to become a better person. Then wait for the Consolation on Easter Sunday.
Then the third event is the Miracle of Healing, which will occur on May 11th, if the Warning, Consolation, and Miracle are this year.
Please take a look at this list of my books and booklets, and see if any topic interests you.