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You must first study and become very familiar with the appropriate standards one requires to meet.
I'm Sorry about providing such generic reply, but EMI is a specialty that requires significant amount of expertise and knowledge.
There are cookbook recipes -for instance- power supplies and video signals, but nevertheless one must still tailor those recipes to the own circuit, and again, to the particular standard one has to meet.
So, first things first....You have to procure the appropriate standard published by the regulatory agency you'll be submitting your samples to. They can be ordered online.
First of all you need to understand how your circuit is working and the the frequencies you need.
Then design it from inside out thinking of removing as much you can of any noise generated from your circuit.
A rule of thumb is that if your circuit don't emit, it will be less susceptible to inbound noise also.
Then you need to add the necessary high voltage/ frequency protection on all in- and outgoing signals.
One of the most critical 'components' are the layout of the PCB, removing any signal loops that can be an antenna for unwanted signals in and out of the circuit.
After your first EMC tests fails, you need to do another round of the above points, and try again.
This goes on until you hopefully get your approvals.