Shan Yu went to Mulan for help to show people he's a good person
Mulan agreed to help Shan Yu but only for 8 months then he will be on his on when they come back to the village but she will always have his friend and have his back.
Mulan taught him how to be polite and use his manners when he's either talking to someone or doing something.
Mulan had worked with him for about 8 months in the forest near water and on the beach to show him he can do it without people around him
Mulan showed him that he had to show everyone respect and have manners so that's what he did when they trained for 8 months. Then they went back and that's when the magic happened, everyone cheered for Shan Yu and how he showed the village the side of him everyone hasn't seen