Be honest, polite, considerate, and straightforward with them. Also, remain open to the possibility, that they may not feel the same about you. The person in question is... well, a person, with free will, and their own individual ways of reacting to people. Some are rude. Some are nice. Some humor you for a while, but do not care sincerely. Some care sincerely, but simply do not share your sentiments. The possibilities are endless, so just be understanding, be yourself, and do not expect to be understood, because not all will understanding.