How?
By your reaction.

Let's use an example....
You're eating breakfast with your family. Your daughter knocks over a cup of coffee onto your business shirt. You have no control over what just happened. What happens next will be determined by how you react. You harshly scold your daughter for knocking the coffee cup over. She breaks down in tears. After scolding her, you turn to your spouse and criticize her for placing the cup too close to the edge of the table. A short verbal battle follows. You storm upstairs and change your shirt. Back downstairs you find your daughter has been too busy crying to finish breakfast and get ready for school - she misses the bus. Your spouse must leave immediately for work. You rush to the car and drive your daughter to school. Because you are late, you drive 40 miles an hour in a 30 mph speed limit.

You have a miserable day at work but are not sure why everything seems to go wrong. You look forward to going home. When you arrive home you find small wedge in your relationship with your spouse and daughter

Why?
Because of how you reacted in the morning.

You had no control over what happened with the coffee. How you reacted in those 5 seconds is what caused your bad day. Here is what could have and should have happened. Coffee splashes over you. Your daughter is about to cry. You gently say "It's OK honey, you just need to be more careful next time".

Grabbing a towel you rush upstairs. After grabbing a new shirt and your briefcase. You come back down in time look through the window and see your child getting on the bus. She turns and waves. You and your spouse kiss before you both go to work. You arrive 5 minutes early and cheerfully greet the staff. Your boss comments on how good of day you are having. Notice the difference. Two different scenarios. Both started the same, but ended differently.

Why?
Because of how you REACTED.

Here are some ways to apply the 90/10 secret:
If someone says something negative about you, do not be a sponge. Let the attack roll off like water on glass. You don't have to let the negative comment affect you! React properly and it will not ruin your day. A wrong reaction could result in losing a friend, being fired, getting stressed out, etc.

How do you react if someone cuts you off in traffic? Do you lose your temper? Pound the steering wheel? Does your blood pressure skyrocket? Do you try and bump them? WHO CARES if you arrive ten seconds later at work? Why let the other driver ruin your day?