how do I change the margins on the second page of a Writer Document so that it only changes the margins on that page?
A key concept in Writer is the use of “page styles” to group pages with some type of formatting. Once you define two page styles with the appropriate format, you need to switch between them, for example by introducing a page break with a change in page style: Insert → More Breaks → Manual Break → select a different page style.
You can learn more about the use of styles and templates in the official documentation or, if you prefer a different approach, in my free book about Writer.