Determine what standards your product needs to meet, or the electromagnetic environment your product will be operated in, and apply design guidelines to minmimize susceptibility to, and generation of that environment. This includes aspects such as layer stackups, trace lengths, PCB ground to equipment chassis and or earth, clock frequencies, use of spread spectrum clocks, cabled I/O impedance, IC decoupling, localized shielding, etc. This is not a trivial subject for the uninitiated - if it matters to the product, seek a mentor or professional assistance.
