What is the basic idea of Kant's Categorical Imperative?

The Categorical Imperative is a test to make sure our actions are moral. Kant states, 'Act only according to that maxim by which you can at the same time will that it should become a universal law.' It is unconditional and independent of circumstances, goals, or desires. The test itself can be written like this: Step 1. Can I will doing 'X' universally? If the answer is Yes then it is morally permissible. If the answer is No then 'X' is wrong. Step 2. Can I will to omit 'X' universally? If the answer is Yes it is a right action but not an obligation. If the answer is No then it is a moral obligation.

reference: Ed L. Miller, Jon Jensen, Questions that Matter: An Invitation to Philosophy, 6th ed., (McGraw-Hill, New York, 2009), pp.405-409.