Don’t call anyone by their first name until they invite you to do so. It is “Professor X” or “Dr. X” until you’ve gotten an email signed with a first name only. Yes, you have just gotten used to calling the professors at your own program by their first names, but guess what? They invited you to.
Ask professors specific questions. Do not ask them to “walk you through” how to do something. Do not assume that just because they wrote or did something that they have no greater joy in life than walking you through said thing, especially when it is obvious you haven’t made an effort to understand it yourself first. How do they know that you’ve made a good faith effort? Ask specific questions!
Many professors cannot read anything longer than three lines. Get to the point quickly. If you are polite and clear, the professor may read more lines, but put them in a new paragraph.