how to i set custom margins

It’s probably right in front of me, but I can’t figure out how to adjust the margin settings for a page.

You set margins by the page layout dialog. Menu item Format - Page, select tab Page.

Note that page settings are not applied to a particular page, but rather to a page style which may be in use for several pages. Typically, when you change margins the new settings will apply to all your pages, because you are using only the default page style for all your pages.

If you need settings to change for one page only, you must use a different page style for that particular page. Depending on context, using a paragraph left/right indent may be a better solution than “locally” changing margins.