Rules of thumb, and some of the reasons for them:
Use some flavor of non-proportional 12-point Courier font. That gives a nice ten characters-per-inch, and it keeps the "i" character the same width as the "w" character. Of course, if the publisher asks for a different font, then you should use whatever they want.
Do not bold and do not use italics. Printing presses use their own system of codes for making something bold or italic--anytime you do it, somebody's going to have to
undo it before the copy can be set. Have you ever seen writers' guidelines which ask that you use a "#" character whenever you want to skip an extra line? That's because the "#" just happens to be the press-code for "put a space here". That gives an extra line which appears to be blank because the only character on it is a space.
Margins should be set to no less than 1" on all four sides. Type should be double-spaced. Editors have a special tool they use. It's called a "blue pencil". It is a pencil. It is blue. The white space between the lines and around the edges allows the editor to make
funny little blue squiggles all over your pretty, clean manuscript.
The average printed page runs about 250 words. Normally, that's not really something the writer needs to worry about.
If the publisher provides guidelines, use them. The strength of your story won't be of much help if your manuscript is deleted, unread.