In the form that opens:
Enter a Budget Name for your billing alert.
Select the appropriate billing account for this budget
Set the Budget Amount. To stay within Google’s free credits, you can set it to $200.
In the Set Budget Alerts section, enter the percentage of the budget at which you want an alert to be triggered. The corresponding spend amount will be calculated automatically.
Alternatively, you can use the default percentages: 50%, 90%, and 100%.