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They set boundaries by saying no. They are able to say no without feeling guilty, because they know what they are saying yes to. The only way to say no to anything is to have a deeper, more important yes. You may think you could never say no and not feel guilty. That may be true in the beginning. But the first step is saying no, even if that means saying no and feeling guilty. Better to say no and feel guilty than to say yes to the wrong things. Emotionally people also understand that other people use guilt to get what they want because it works, and they take themselves out of those manipulations by saying no.
Emotionally people realize everyone is carry a heavy burden. They have large stores of empathy and give people the benefit of the doubt.