To "get fancy" is to make something complicated and/or impress others. If someone says, "Don't you get fancy with me," or "You don't need to get fancy about it," that means don't try to impress me. Another way to say this is, "Don't try to show off." The person who says this is likely annoyed and does not like the actions of the person who is being fancy/showing off. "If you want to get fancy with it" is often used for humour by trying to explain something more complexly than necessary. Instead, you can say, "If I need to explain this in detail..." or "If I need to do this the complicated way..." Let's first break it down to what is "fancy" in English so you can see where these sentences and ideas come from. The adjective form of fancy usually means something that is high quality, unusually complicated, and/or with the intention of impressing other people. Then there is the verb form of "fancy," which has many meanings. [Fancy something] means to want something or want to do something. [Fancy someone] means you are romantically or sexually attracted to someone. Another meaning, [fancy yourself], is to think you are popular, attractive, or intelligent. This last meaning is closest to the sentences in your post. If someone is getting fancy with their own actions, they likely think they are good or more intelligent and want to impress someone. I hope this all helps!