Broadly speaking, showing consideration and regard for someone or something constitutes respectful behavior.  Unfortunately, the interpretation of what constitutes consideration is subjective.

A lack of consideration and regard is not necessarily disrespectful, pro-actively behaving in a way that clearly demonstrates derision is.  As noted though, the interpretation of derision or disrespect can be subjective.

Respectful, why bother

In recent years many people have indicated they find the behavior of the current U.S. president (Donald Trump) less than respectful.  His supporters argue his directness (apparent disrespect to others) is one of the key reasons he achieves his goals.

Perhaps politicians have a higher bar for respectful behavior and Tumps behavior is more acceptable for a businessman than a politician (although subjectively I would question that as well).

If Trumps behavior is not at the level of respect expected for someone in his position (i.e., POTUS) but he is achieving his goals and applauded by his supporters, why should he bother being more respectful?

In fact, it appears he is rewarded for being abusive.

How to behave respectfully

Although respect has a subjective component, this is not a fair excuse for not acting in a respectful way.  We constantly receive feedback from others (both direct and indirect).

Respect simply requires us to be aware of this feedback and react accordingly.  Others constantly send messages about how they are feeling (happy/unhappy, comfortable/uncomfortable, confused/clear, etc).

Sensitivity to this feedback (regardless of our subjective interpretation of the context) and responding accordingly to create a positive context is all that is needed to be perceived as respectful.  It is that simple.